In the opening scene of Lena Dunham's HBO series Girls, the Horvaths tell their 24-year-old daughter Hannah that they will no longer support her—or, as her mother puts it: "No. More. Money." A recent college graduate, Hannah has been living in Brooklyn, completing an unpaid internship and working on a series of personal essays. The Horvaths intend to give Hannah "one final push" toward, presumably, a lifestyle that more closely resembles adulthood. Hannah protests. Her voice quavers. She tells her parents that she does not want to see them the following day, even though they are leaving town soon: "I have work and then I have a dinner thing and then I am busy—trying to become who I am."

Across the United States—and in developed nations around the world—twenty-somethings like Hannah are taking longer to finish school, leave home, begin a career, get married and reach other milestones of adulthood. These trends are not just anecdotal; sociologists and psychologists have gathered supporting data. Robin Marantz Henig summarizes the patterns in her 2010 New York Times Magazine feature:

"One-third of people in their 20s move to a new residence every year. Forty percent move back home with their parents at least once. They go through an average of seven jobs in their 20s, more job changes than in any other stretch. Two-thirds spend at least some time living with a romantic partner without being married. And marriage occurs later than ever. The median age at first marriage in the early 1970s, when the baby boomers were young, was 21 for women and 23 for men; by 2009 it had climbed to 26 for women and 28 for men, five years in a little more than a generation."

These demographic shifts have transformed the late teens through mid twenties into a distinct stage of life according to Jeffrey Arnett of Clark University, who calls the new phase "emerging adulthood." Arnett acknowledges that emerging adulthood is relevant only to about 18 percent of the global population, to certain groups of twenty-somethings in developed nations such as the United States, Canada, Western Europe, Japan and Australia. To make some broad generalizations, people living in the rest of world—particularly in developing countries—are much more likely to finish formal education in their teens and marry by their early twenties.

Although Arnett primarily studies how society, culture and the economy have created emerging adulthood, some scientists and journalists have wondered whether biology is involved as well. Henig writes that some researchers think a lengthy preamble to adulthood might be "better-suited to our neurological hard-wiring" and that the general ambivalence of twenty-somethings—feeling that they are sort of adults, but not really adults— "reflects what is going on in the brain, which is also both grown-up and not-quite-grown-up." Most recently, The Wall Street Journal ran an article recommending that concerned parents of twenty-somethings "chill out" because "recent research into how the brain develops suggests that people are better equipped to make major life decisions in their late 20s than earlier in the decade. The brain, once thought to be fully grown after puberty, is still evolving into its adult shape well into a person's third decade, pruning away unused connections and strengthening those that remain, scientists say."

After reading The Wall Street Journal article, a flock of questions began flapping in my own twenty-something mind. What does it mean to have a "fully grown" adult brain anyways and, if my peers and I do not yet have such a brain, exactly how un-grown-up are our noggins, how uncooked our noodles? Were we neurologically unfit to make the important decisions about careers and marriage that some of us have already made? If emerging adulthood is itself an emerging phenomenon, what is its precise relationship to the biology of an organ whose defining characteristics began evolving millions of years ago? And does a Peter Pan brain have any redeeming qualities?

Budding brains

In an ongoing study that kicked off in 1991, Jay Giedd of the National Institute of Mental Health has been tracking the brain development of nearly 4,000 people ranging in age from a few days to 96 years. Every two years, Giedd invites his volunteers to the lab to scan their brains with magnetic resonance imaging (MRI). Giedd and his colleagues have learned that, contrary to neuroscientists' earliest assumptions, the brain continues to markedly rewire itself even after puberty.

Between 12 and 25, the brain changes its structure in a few important ways. Like an overeager forest, neurons in the early adolescent brain become bushier, growing more and more overlapping branches whose twigs reach toward one another, nearly touching except for tiny gaps known as synapses. When an electrical impulse—or action potential—reaches a twig, the neuron flings spurts of chemical messages across the synapse. Over time, depending on how teens busy their minds, twigs around the least used synapses wither, while twigs flanking the most trafficked synapses grow thicker, strengthening those connections. Meanwhile, as neurons in the adolescent brain make and break connections, glia—non-firing brain cells—set to work wrapping neurons in a fatty white tissue known as myelin, considerably increasing the speed at which electrical impulses travel along neurons' branches.

Although these developmental changes continue far longer than researchers initially thought, they are not as dramatic in the twenties as they are in the teens. "In the twenties, the brain is definitely still changing, but it's not rampant biological change," explains Beatriz Luna of the University of Pittsburgh. "Most of the brain's systems are good to go in one's twenties." In an email message, B.J. Casey of Weill Cornell Medical College made a similar remark: "Most of my functional imaging work shows the greatest brain changes between 13 and 17 with relative stability across 20s."

In her own studies, Luna has found that, at least on certain cognitive tasks, people in their early twenties perform just as well as people in their late twenties. She often asks her volunteers to deliberately look away from a flashing light on a screen—a test of impulse inhibition, since flickers attract our attention. "Ten-year-olds stink at it, failing about 45 percent of the time," as David Dobbs put it in his National Geographic feature. "Teens do much better. In fact, by age 15 they can score as well as adults if they're motivated, resisting temptation about 70 to 80 percent of the time…And by age 20, their brains respond to this task much as the adults' do."

In Luna's studies, brain behavior changed in parallel with improving scores. Older volunteers showed higher activity in brain regions involved in identifying errors, such as the anterior cingulate cortex. Related research has shown that older adolescents have stronger bridges of neural tissue connecting the emotional and motor centers of their brains with the prefrontal cortex, an "executive" brain region known for, among many other things, inhibiting impulses and tempering bubbling emotions. Luna and other researchers now think that, more than the growth of any single brain region, this increasing interconnectedness characterizes brain development in the twenties. Of course, that doesn't mean that once someone leaves behind their twenties they will never again lose their cool or act thoughtlessly instead of prudently. Individual variation makes all the difference. Some teens and twenty-somethings are simply more cautious and composed than some adults.

To reflect the ongoing structural changes in the adolescent and twenty-something brain, many journalists and scientists use words and phrases like "unfinished," "work in progress," "under construction" and "half-baked." Such language implies that the brain eventually reaches a kind of ideal state when it is "done." But there is no final, optimal state. The human brain is not a soufflé that gradually expands over time and finally finishes baking at age 30. Yes, we can identify and label periods of dramatic development—or windows of heightened plasticity—but that should not eclipse the fact that brain changes throughout life.

Studies have confirmed, for example, that London taxicab drivers grow larger hippocampi as they learn to navigate London's convoluted roadways. This growth in the hippocampus, a brain region essential for forming new memories, is not explained by youth: According to the Public Carriage Office, 98 per cent of London taxi drivers are over the age of 30 [PDF]. Granted, the hippocampus is one of only two regions thought to grow new neurons in adulthood, but the brain remains remarkably plastic in other ways too. When one part of the brain shrivels—say, from stroke or traumatic injury—nearby regions often adopt their deceased neighbor's functions. When blind people learn to use echolocation, areas of their brains usually devoted to vision learn to interpret the echoes of clicks and taps instead. Neuroplasticity is an everyday phenomenon as well. The adult brain constantly strengthens and weakens connections between its cells. In fact, learning and memory are dependent on such flexibility. Learning a new language or picking up an instrument may be easier when one is young, but adaptability and creativity do not expire on one's 30th birthday.

Brains of our past and present

Mapping structural changes in the brain over time tells us how the brain matures, but not why it matures that way. To answer why we have to think about the benefits that prolonged brain development would have offered our ancestors. After all, the human brain's fundamental phases of development could not have popped into existence in the last 50 years or even thousands of years ago—more likely, they evolved at least a couple million years ago in the Paleolithic, when the human brain began to increase in size and morph into the organ we know today. Keeping the brain extra flexible for a longer period of time may have provided our ancestors with more opportunities to quickly master new skills and adapt to a changing environment. But taking too long to learn how to manage emotions, control impulses and plan ahead may also have impeded survival.

Painting an accurate tableau of the Paleolithic lifestyle is difficult because the evidence is scant, but we can say a few things with confidence. First, although the exact lifespans of early humans are not certain, evidence from the fossil record—as well as death rates among modern hunter-gatherer societies—suggests that most early humans did not live as long as people in developed nations today. Children frequently died in their first years of life. If you made it to 15, you were more likely to live at least another 15 or 20 years, but people who lived past their forties were probably in a minority. Second, early humans likely started having children far sooner than people in industrialized countries today. Paleolithic twenty-somethings, we can safely assume, did not have the luxury of spending a few years after college "finding themselves" while backpacking through India and volunteering on organic farms. Rather, people who survived to their twenties in the Paleolithic probably had to bear the responsibilities of parenthood as well as contribute substantially to their community's survival. These are not exactly circumstances that favor leisurely cognitive development late into one's twenties.

When I described this scenario to Giedd, however, he suggested that widening the window of heightened neuroplasticity to encompass one's twenties may have helped Homo sapiens adapt to rapid shifts in the climate. Unfortunately, as with many hypotheses in evolutionary psychology, scientists do not have a way to objectively test these ideas. Still, if we want to fully understand the brain, we cannot ignore the fact that it evolved in circumstances very different from our own.

For now, let's put the brains of ancient twenty-somethings out of our minds. What about the twenty-somethings of today? Even if the brain's developmental changes are more dramatic in the teens than in the twenties, the best available evidence suggests that a twenty-something's brain boasts a little more adaptability than an older brain. Our twenties might represent a final opportunity to begin mastering a particular skill with a kind of facility we cannot enjoy in later decades. Should people in their twenties buckle down and choose something, anything, to practice while their brains are still nimble? Does the neuroscience suggest that, for all their freedom and fun, gallivanting twenty-somethings neglect their last years of heightened plasticity? Should parents encourage their 20-year-olds to shirk adult responsibilities lest they hamper an advantageous period of self-discovery and wild experimentation?

Solid neuroscience that can directly answer these questions does not yet exist. "It's too soon to tell," Giedd says, "but we're wondering." He and his colleagues plan to compare the brain development of girls who become pregnant in their teens to girls who do not. "Teen pregnancy changes all your priorities and what you do with your time—how do those experiences change the brain?" Arnett agrees that such neuroimaging studies would be useful. "Even in industrialized countries, a lot of people still get married pretty early. You could do brain studies comparing people who experience their twenties differently and contrast how their brains develop."

In the meantime, twenty-somethings can expect increasingly frequent waves of sage advice from academics, bloggers and concerned parents alike. "Watching talking cats on YouTube isn't as good for cognitive development as reading or taking classes," Laurence Steinberg of Temple University told The Wall Street Journal. Truth. In the same article, Jennifer Tanner, co-chair of the Society for the Study of Emerging Adulthood, provides her own pearl: "My advice is, if your parents are currently doing things for you that you could do for yourself, take the controls. Say, 'No. Mom, Let me get my own shampoo.'" Thanks for the tip, Ms. Tanner. I mean, if I were living at home to save money, I wouldn't mind sharing the jumbo size 2-in-1 shampoo and conditioner with my siblings. But I'm pretty sure the vast majority of my peers have a handle on shampoo selection by now. Because we're worth it.

Emerging adulthood is real—it's happening, albeit to a small percentage of the world's population. Whether we can, at this moment in time, meaningfully link this life stage to neuroscience seems a tenuous proposition at best. By itself, brain biology does not dictate who we are. The members of any one age group are not reducible to a few distinguishing structural changes in the brain. Ultimately, the fact that a twenty-something has weaker bridges between various brain regions than someone in their thirties is not hugely important—it's just one aspect of a far more complex identity.