You’re not getting any younger—though you probably don’t need to be reminded of that fact. Aging can be exciting, scary, sad, and just about every feeling in between. Needless to say, moving through the stages of life is a complex process—emotionally and physically.

As you age, you pass from infancy, adolescence, emerging adulthood, and eventually to middle age and beyond. These stages of life bring their own sets of challenges and rewards. But what exactly sets them apart? What is adolescence? What is adulthood? When do those transitions between stages happen? And when does adulthood become middle age?

Keep reading for the answers to these questions and more. By the end, you’ll have broken down adolescence, adulthood, and middle age. And you’ll be able to explain the characteristics and defining features of the stages of life.

What is Adolescence: It’s More Than Just the Teenage Years

Most people think of adolescents and teenagers as more or less the same group. And while there is a lot of overlap, there are differences. The teenage years begin at 13 and end at 19. Adolescence, on the other hand, starts somewhere around age 10 and continues until your early twenties.

This broadness is necessary because adolescence refers to the transition from childhood to adulthood—physically and mentally. This is a time of drastic physical, emotional, and social development. Because the adolescent stage of life spans over a decade, it is often broken into three phases: early, middle, and late adolescence. And each phase is characterized by its own set of changes and development:

Early adolescence (ages 10-13): This phase of adolescence is responsible for those oh-so-hated middle school years. Growth spurts typically begin during this time (especially for girls), as do other physical changes—like the growth of body hair and development of primary and secondary sex characteristics. These sudden, often drastic, changes can lead pre-teens to feel awkward or uncomfortable in their bodies.
While these physical changes are the scientific markers of early adolescence, there are also many mental changes and developments common in this stage of life. During early adolescence, preteens and teens often begin to develop a stronger sense of self. This often includes testing the limits of their independence and pushing for more privacy and self-determination.

In other words, during early adolescence, most individuals want to make choices for themselves. It could be what they wear, eat, or how their room looks. As early adolescents begin to form opinions, they often exhibit black-and-white thinking.

Middle adolescence (ages 14-17): There’s no clear line between early, middle, and late adolescence because everybody develops on a different timeline. But some developmental patterns hold true generally. Girls, for example, often hit their growth spurts in early adolescence, while many boys don’t have theirs until middle adolescence. And so much of middle adolescence, from a physical-development standpoint, can be described as “more of the same.”

During middle adolescence, teens experience rapid cognitive development. Though the brain won’t be fully developed for several more years, certain brain functions—such as logical reasoning—reach maturity by age 16. This means teens are just as capable of logical reasoning as adults. (Whether their ability to think logically translates into rational behavior or not is another story.)

Middle adolescence is also a time when many teens begin to explore romantic relationships. This often takes the form of dating. And teens may also begin to question and explore their sexuality to create and understand their own sexual identity.

All of these changes go hand-in-hand with teens’ desire for independence that typically grows stronger with age. And for teens in the United States, middle adolescence often brings the keys (pun intended) to that ultimate form of independence: a driver’s license.

Late adolescence (ages 18-21): If you’re surprised to see 21 included under adolescence, don’t worry—you’re probably not the only one! Teens legally become adults at age 18, so people tend to think this is when adolescence ends. In terms of development and growth, however, humans don’t reach adulthood until sometime in their twenties.

By late adolescence, most of your physical growth is out of the way, so the development that occurs during this stage is mostly cognitive. Teens are infamous for taking risks, but as they age, they improve at thinking ahead. This is all thanks to brain development.

The prefrontal cortex, which is the last part of the brain to develop, has just about reached its adult form by late adolescence. And the prefrontal cortex is responsible for decision making, among other key operations. (It’s important to note that the brain is still only mostly developed. It won’t reach full maturity until age 25 or 26.)

In this final stage of adolescence, the beliefs, identity, and values that began to develop in earlier years also become far more stable. They may change with experience, of course, but as you transition into adulthood these aspects of your identity are often what keep you rooted.

What is Adulthood and How to Tell When Adolescence Officially Ends

Kids and teens want more than anything to be a grown up. This raises a question without an easy answer: what exactly is a grown up? Ask any adult and they probably won’t give you a straight answer. Legally, anyone older than 18 is an adult in the U.S. But, as mentioned above, this cutoff doesn’t make much sense from a developmental standpoint.

So where can we draw the line between the adolescent and adulthood stages of life?

By age 22, the body is almost entirely done growing. Your bones (except for your collarbone, which won’t fully mature until your early-to-mid thirties) are the size they’ll be for the rest of your adult life. Your wisdom teeth have come in. And your brain is finishing up its final prefrontal cortex development.

From this time until middle age, your body operates at its peak physical performance. This doesn’t mean you’ll be in the best shape of your life. But it does mean your heart, lungs, muscles, and other organs will be operating at their most efficient and effective levels.

With these physical developments out of the way, adulthood brings a whole new set of developmental challenges: as a young adult, you will develop the social, emotional, and lifestyle habits that shape the rest of your life. No pressure, right?

Don’t worry, these changes don’t happen overnight. You’re not an adolescent one day and a full-fledged adult the next. It’s a gradual process. For this reason, many scientists have defined a new intermediary stage of life development: early or emerging adulthood.

What is emerging adulthood: Emerging adulthood is a term some researchers use to describe the transitional phase between adolescence and adulthood. It is not a developmental stage, per se, but rather a period of social and emotional exploration and growth. Emerging adulthood is a time when many young adults begin to explore various jobs, establish their first serious romantic relationships, and navigate the new set of challenges “adulthood” brings. (These challenges include living independently, finding a career, and, in some cases, becoming a parent or spouse.)

It is nearly impossible to pin down exact age boundaries for emerging adulthood because this stage varies widely from person to person. Some may settle into adulthood by 25, while others are still struggling to establish their adult identity well into their late twenties. Which is to say, describing adulthood isn’t an exact science.

Consider the question posed at the start of this section again: what exactly is a grown up? Or, in other words, what is adulthood? At this point, you’ve been given two ways to answer this question. You can approach it from a developmental standpoint or from a social and emotional standpoint.

The first provides a much more clear-cut answer: adulthood begins around age 22, when the physical developments that characterize adolescence are complete. But, as outlined above, this approach doesn’t always give a satisfying answer. It seems to only capture part of the picture. Most of all, it doesn’t pin down what adulthood means.

To capture the full picture of adulthood, you also need to consider social factors. Is having a job part of adulthood? Living independently? Having stable romantic relationships? You’ll have to answer those questions for yourself.

So what is adulthood? Sometimes the best answer might be “I know it when I see it.”

Middle Age: It’s Not All Downhill From Here

Emerging adulthood may be a vague, loosely defined stage of life. But middle age has clearer boundaries. It begins at 40 and ends at 65—give or take a few years on either end.

During this period, most people’s lives have stabilized in comparison to their twenties and thirties. By middle age, you’ll likely have settled into a career or career field, become comfortable with your beliefs and values, and are maintaining long-term, stable friendships and relationships. If early adulthood is a time of exploration and discovery, middle age is a time of settling in.

As you approach and move through middle age, you might find yourself frequently thinking, “I’m not as young as I used to be”—especially while exercising. People tend to hit peak physical performance in early adulthood. From there on out, the wear and tear on your body will become more apparent. It’s a natural part of aging.

This manifests itself in several ways. You might find yourself bouncing back from injuries much more slowly or getting winded faster. Additionally, risk for certain health conditions becomes much higher in middle age. Screening for these conditions can help detect them early—and early detection is a crucial part of treatment. After the age of 40, you should start screening for high blood pressure, diabetes, and heart disease. You may also screen for breast and cervical cancer if you are a woman, and prostate cancer if you are a man.

This might sound like a drag, but life is not all downhill from here! Studies show that happiness and life satisfaction bottoms out somewhere between ages 45 and 50—right as you are hitting middle age. From there on out, the older you get, the happier and more satisfied you will probably be.

Take Care of Your Health at Every Stage of Life

Every stage of life comes with unique challenges and rewards. So whether you’re a teen eager to grow up or an adult wishing you could turn back the clock, remember to focus on the present. Take care of your health no matter your age. It’s never too early or too late to make your lifestyle a little bit healthier. And take time to appreciate the joys each stage of life has to offer!

Growing older is a natural phase of life. It follows then that as you age, your cells age, too. And in fact, cellular aging is a simple fact of biology, but one that needn’t be shrouded in mystery.

Cellular aging mechanisms are in place from the day you are born. As cells divide, multiply, and perform their designated functions, they age. And as they age, your body has in place remarkable ways to take care of aging cells and replenish them with new ones.

So, what causes cell aging anyway? Here are some of the most common triggers of cell aging:

• DNA damage
• Oxidative stress (from internal and external sources)
• Decline in autophagy

It’s important to remember that your aging body and older cells aren’t something to shy away from. You aren’t only getting older; your body is signaling to the world what a wonderful life you have lived.

And as for your cells—aging is just another period in their much more microscopic lifecycle. Show your older cells some deference and learn more about their unique aging process.

Cellular Aging—Definitions and Mechanisms

In scientific literature, aging is referred to as senescence. Cellular senescence, specifically, is the process of cellular aging. A senescent cell is generally larger than its non-senescent counterparts. Senescent cells no longer divide in an effort to protect themselves and the tissue surrounding them from inaccurate or harmful replication errors. The process by which a replicating cell transforms into a non-dividing senescent cell takes about six weeks to complete.

DNA replication is at the heart of cellular senescence. In order to maintain healthy, functional tissues and organs, the cells involved need to replicate without error. Your body has natural triggers in place to manage when older cells become senescent and no longer replicate. Aging triggers come from within the senescent cell and the environment around them.

You already read about the three common causes of cell aging, now it’s time to dive deeper into each one.

DNA Damage

New cells don’t need to worry too much about damage to their DNA. The chromosomes that store all your unique genetic information are capped with sections of repeating genetic code that signifies the end of a chromosome. These chromosome caps are called telomeres and they help maintain reliabil and accuracy during DNA replication.

But with each cycle of replication—every time a cell divides, and as the cell ages—a small percentage of the genetic code is lost, and the telomere caps shorten. As the cell ages and telomeres shorten, the cell is more likely to experience damage to its DNA or incorrect replication.

To preserve the integrity of your genetic code the telomeres at the ends of each chromosome signal when it’s time for the cell to stop replicating. Without the telomere caps, gene transcription and cell division would continue indefinitely—leading to a potentially dangerous accumulation of poorly made cells. Your cells rely on telomeres to know when it’s time to retire.

Oxidative Stress

This is another event that triggers cell aging. And oxidative stress can also halt cell replication. Reactive oxygen species in the cell’s environment are fodder for DNA replication mishaps. They can lead to mutations in cell’s genetic code that may affect the function and health of the cell over time.

When reactive oxygen species are detected in the cell’s environment, replication stops in order to preserve the integrity of the cell’s DNA. Aging cells that stop replicating in the presence of reactive oxygen species are protecting your body from incorrect cell proliferation and mistakes in gene transcription.

Decline in Autophagy

Kudos to you if you can recall the definition of this scientific term. Autophagy literally means “self-eat.” And this simple phrase perfectly describes how autophagy is used by cells. As cells age, their organelles (cell parts) and cellular equipment begin to fail. Waste can build up, and it needs to be cleared away. Autophagy is the cell’s way of destroying used and broken parts through a process of self-digestion.

Specialized organelles inside your cells collect damaged cellular material and break it down. These organelles are called lysosomes. They are full of digestive enzymes that eliminate the junk that can build up in your cells.

A cell’s ability to perform autophagy dwindles with age, creating a struggle to clean house when broken-down organelles and waste pile up. This can lead to an accumulation of proteins within the aging cell and may trigger problems with DNA replication down the line.

When a cell can no longer manage the buildup of waste within its cell membrane, it stops dividing and triggers senescence.

Apoptosis vs Cellular Senescence

If you research cell aging long enough, you’ll likely come across a phenomenon called apoptosis. This cellular process is easily confused with senescence, so let’s clear the air on the circumstances that lead to each.

Like you’ve read above, cellular senescence is the end of cell division for the aging cell. A senescent cell continues to perform its original function, but it no longer replicates—to avoid mistakes in genetic transcription. Aging cells aren’t dead cells, but they are less productive and efficient than younger, replicating cells.

Apoptosis is essentially programmed cell death. Sometimes during DNA replication, a cell can stray far from its prescribed course. Uncontrollable replication can lead to abnormal cell growth, a potentially harmful buildup of poorly manufactured cell copies. To stop this overgrowth dead in its tracks, cells have a special self-destruct protocol they can follow.

Older cells are more likely to apoptose, but that doesn’t necessarily mean all senescent cells are headed for immediate self-destruction. When apoptosis is triggered, the cell releases proteins that neatly pack up all the inner workings of the cell and cause it to lyse (pop). Apoptosis isn’t messy, and cells undergoing apoptosis don’t harm their neighboring healthy cells.

In summary, cellular senescence stops cell division and apoptosis occurs when an aging cell can’t stop dividing. Hopefully this interlude clears up some of the confusion surrounding the topic of cell aging.

Healthy Living and Cellular Aging

Aging cells are a fact of life. As your cells age, your body replaces them with young, high-performing cells to take over when older cells retire. No matter the stage of your life or your cell’s lifecycle, you can promote cellular and whole-body wellness with healthy living.

Cellular senescence is unavoidable, but you can protect healthy cells from entering retirement too early. Some activities can shorten telomeres and trigger premature cell aging. Do your best to avoid these:

• Sunburns
• Tanning beds
• Smoking
• Drug abuse
• Obesity
• Excess stress

These habits have been shown to elevate oxidative stress from reactive oxygen species—especially tanning and sunburn. And as you know, reactive oxygen species are one of the triggers of cellular senescence.

One way to protect your cells and support them as they age is by maintaining good cellular health habits. You know how much healthy habits help you feel your best. There are lifestyle and diet choices that can optimize cellular your health, too. Take a minute to review four key habits and learn how to keep your cells healthy.

And remember, aging bodies and aging cells are natural. This latter period of life is meant to be enjoyed. So celebrate aging bodies and aging cells with gratitude and respect for all they’ve accomplished. Pay respect to your body as you and your cells age by avoiding the triggers of cellular aging and supporting healthy cells with a diet rich in antioxidants and other cell-supporting habits.