But Arnett said “young adulthood” was too broad a term to apply to a 25-year span that included both him and his college students. The 20s are something different from the 30s and 40s, he remembered thinking. And while he agreed that the struggle for intimacy was one task of this period, he said there were other critical tasks as well.

Arnett and I were discussing the evolution of his thinking over lunch at BABA Sushi, a quiet restaurant near his office where he goes so often he knows the sushi chefs by name. He is 53, very tall and wiry, with clipped steel-gray hair and ice-blue eyes, an intense, serious man. He describes himself as a late bloomer, a onetime emerging adult before anyone had given it a name. After graduating from Michigan State University in 1980, he spent two years playing guitar in bars and restaurants and experimented with girlfriends, drugs and general recklessness before going for his doctorate in developmental psychology at the University of Virginia. By 1986 he had his first academic job at Oglethorpe University, a small college in Atlanta. There he met his wife, Lene Jensen, the school’s smartest psych major, who stunned Arnett when she came to his office one day in 1989, shortly after she graduated, and asked him out on a date. Jensen earned a doctorate in psychology, too, and she also teaches at Clark. She and Arnett have 10-year-old twins, a boy and a girl.

Arnett spent time at Northwestern University and the University of Chicago before moving to the University of Missouri in 1992, beginning his study of young men and women in the college town of Columbia, gradually broadening his sample to include New Orleans, Los Angeles and San Francisco. He deliberately included working-class young people as well as those who were well off, those who had never gone to college as well as those who were still in school, those who were supporting themselves as well as those whose bills were being paid by their parents. A little more than half of his sample was white, 18 percent African-American, 16 percent Asian-American and 14 percent Latino.

More than 300 interviews and 250 survey responses persuaded Arnett that he was onto something new. This was the era of the Gen X slacker, but Arnett felt that his findings applied beyond one generation. He wrote them up in 2000 in American Psychologist, the first time he laid out his theory of “emerging adulthood.” According to Google Scholar, which keeps track of such things, the article has been cited in professional books and journals roughly 1,700 times. This makes it, in the world of academia, practically viral. At the very least, the citations indicate that Arnett had come up with a useful term for describing a particular cohort; at best, that he offered a whole new way of thinking about them.

DURING THE PERIOD he calls emerging adulthood, Arnett says that young men and women are more self-focused than at any other time of life, less certain about the future and yet also more optimistic, no matter what their economic background. This is where the “sense of possibilities” comes in, he says; they have not yet tempered their ideal­istic visions of what awaits. “The dreary, dead-end jobs, the bitter divorces, the disappointing and disrespectful children . . . none of them imagine that this is what the future holds for them,” he wrote. Ask them if they agree with the statement “I am very sure that someday I will get to where I want to be in life,” and 96 percent of them will say yes. But despite elements that are exciting, even exhilarating, about being this age, there is a downside, too: dread, frustration, uncertainty, a sense of not quite understanding the rules of the game. More than positive or negative feelings, what Arnett heard most often was ambivalence — beginning with his finding that 60 percent of his subjects told him they felt like both grown-ups and not-quite-grown-ups.

Some scientists would argue that this ambivalence reflects what is going on in the brain, which is also both grown-up and not-quite-grown-up. Neuroscientists once thought the brain stops growing shortly after puberty, but now they know it keeps maturing well into the 20s. This new understanding comes largely from a longitudinal study of brain development sponsored by the National Institute of Mental Health, which started following nearly 5,000 children at ages 3 to 16 (the average age at enrollment was about 10). The scientists found the children’s brains were not fully mature until at least 25. “In retrospect I wouldn’t call it shocking, but it was at the time,” Jay Giedd, the director of the study, told me. “The only people who got this right were the car-rental companies.”

When the N.I.M.H. study began in 1991, Giedd said he and his colleagues expected to stop when the subjects turned 16. “We figured that by 16 their bodies were pretty big physically,” he said. But every time the children returned, their brains were found still to be changing. The scientists extended the end date of the study to age 18, then 20, then 22. The subjects’ brains were still changing even then. Tellingly, the most significant changes took place in the prefrontal cortex and cerebellum, the regions involved in emotional control and higher-order cognitive function.