If society decides to protect these young people or treat them differently from fully grown adults, how can we do this without becoming all the things that grown children resist — controlling, moralizing, paternalistic? Young people spend their lives lumped into age-related clusters — that’s the basis of K-12 schooling — but as they move through their 20s, they diverge. Some 25-year-olds are married homeowners with good jobs and a couple of kids; others are still living with their parents and working at transient jobs, or not working at all. Does that mean we extend some of the protections and special status of adolescence to all people in their 20s? To some of them? Which ones? Decisions like this matter, because failing to protect and support vulnerable young people can lead them down the wrong path at a critical moment, the one that can determine all subsequent paths. But overprotecting and oversupporting them can sometimes make matters worse, turning the “changing timetable of adulthood” into a self-fulfilling prophecy。

　　The more profound question behind the scholarly intrigue is the one that really captivates parents: whether the prolongation of this unsettled time of life is a good thing or a bad thing. With life spans stretching into the ninth decade, is it better for young people to experiment in their 20s before making choices they’ll have to live with for more than half a century? Or is adulthood now so malleable, with marriage and employment options constantly being reassessed, that young people would be better off just getting started on something, or else they’ll never catch up, consigned to remain always a few steps behind the early bloomers? Is emerging adulthood a rich and varied period for self-discovery, as Arnett says it is? Or is it just another term for self-indulgence?

　　THE DISCOVERY OF adolescence is generally dated to 1904, with the publication of the massive study “Adolescence,” by G. Stanley Hall, a prominent psychologist and first president of the American Psychological Association. Hall attributed the new stage to social changes at the turn of the 20th century. Child-labor laws kept children under 16 out of the work force, and universal education laws kept them in secondary school, thus prolonging the period of dependence — a dependence that allowed them to address psychological tasks they might have ignored when they took on adult roles straight out of childhood. Hall, the first president of Clark University — the same place, interestingly enough, where Arnett now teaches — described adolescence as a time of “storm and stress,” filled with emotional upheaval, sorrow and rebelliousness. He cited the “curve of despondency” that “starts at 11, rises steadily and rapidly till 15 . . . then falls steadily till 23,” and described other characteristics of adolescence, including an increase in sensation seeking, greater susceptibility to media influences (which in 1904 mostly meant “flash literature” and “penny dreadfuls”) and overreliance on peer relationships. Hall’s book was flawed, but it marked the beginning of the scientific study of adolescence and helped lead to its eventual acceptance as a distinct stage with its own challenges, behaviors and biological profile。

　　n the 1990s, Arnett began to suspect that something similar was taking place with young people in their late teens and early 20s. He was teaching human development and family studies at the University of Missouri, studying college-age students, both at the university and in the community around Columbia, Mo. He asked them questions about their lives and their expectations like, “Do you feel you have reached adulthood?”

　　“I was in my early- to mid-30s myself, and I remember thinking, They’re not a thing like me,” Arnett told me when we met last spring in Worcester. “I realized that there was something special going on。” The young people he spoke to weren’t experiencing the upending physical changes that accompany adolescence, but as an age cohort they did seem to have a psychological makeup different from that of people just a little bit younger or a little bit older. This was not how most psychologists were thinking about development at the time, when the eight-stage model of the psychologist Erik Erikson was in vogue. Erikson, one of the first to focus on psychological development past childhood, divided adulthood into three stages — young (roughly ages 20 to 45)，天剑狂刀私服， middle (about ages 45 to 65) and late (all the rest) — and defined them by the challenges that individuals in a particular stage encounter and must resolve before moving on to the next stage. In young adulthood, according to his model, the primary psychological challenge is “intimacy versus isolation,” by which Erikson meant deciding whether to commit to a lifelong intimate relationship and choosing the person to commit to。