We’re in the thick of what one sociologist calls “the changing timetable for adulthood。” Sociologists traditionally define the “transition to adulthood” as marked by five milestones: completing school, leaving home, becoming financially independent, marrying and having a child. In 1960, 77 percent of women and 65 percent of men had, by the time they reached 30, passed all five milestones. Among 30-year-olds in 2000, according to data from the United States Census Bureau, fewer than half of the women and one-third of the men had done so. A Canadian study reported that a typical 30-year-old in 2001 had completed the same number of milestones as a 25-year-old in the early ’70s。

　　The whole idea of milestones, of course, is something of an anachronism; it implies a lockstep march toward adulthood that is rare these days. Kids don’t shuffle along in unison on the road to maturity. They slouch toward adulthood at an uneven, highly individual pace. Some never achieve all five milestones, including those who are single or childless by choice, or unable to marry even if they wanted to because they’re gay. Others reach the milestones completely out of order, advancing professionally before committing to a monogamous relationship, having children young and marrying later, leaving school to go to work and returning to school long after becoming financially secure。

　　Even if some traditional milestones are never reached, one thing is clear: Getting to what we would generally call adulthood is happening later than ever. But why? That’s the subject of lively debate among policy makers and academics. To some, what we’re seeing is a transient epiphenomenon, the byproduct of cultural and economic forces. To others, the longer road to adulthood signifies something deep, durable and maybe better-suited to our neurological hard-wiring. What we’re seeing, they insist, is the dawning of a new life stage — a stage that all of us need to adjust to。

　　JEFFREY JENSEN ARNETT, a psychology professor at Clark University in Worcester, Mass., is leading the movement to view the 20s as a distinct life stage, which he calls “emerging adulthood。” He says what is happening now is analogous to what happened a century ago, when social and economic changes helped create adolescence — a stage we take for granted but one that had to be recognized by psychologists, accepted by society and accommodated by institutions that served the young. Similar changes at the turn of the 21st century have laid the groundwork for another new stage, Arnett says, between the age of 18 and the late 20s. Among the cultural changes he points to that have led to “emerging adulthood” are the need for more education to survive in an information-based economy; fewer entry-level jobs even after all that schooling; young people feeling less rush to marry because of the general acceptance of premarital sex, cohabitation and birth control; and young women feeling less rush to have babies given their wide range of career options and their access to assisted reproductive technology if they delay pregnancy beyond their most fertile years。

　　Just as adolescence has its particular psychological profile, Arnett says, so does emerging adulthood: identity exploration, instability, self-focus, feeling in-between and a rather poetic characteristic he calls “a sense of possibilities。” A few of these, especially identity exploration, are part of adolescence too, but they take on new depth and urgency in the 20s. The stakes are higher when people are approaching the age when options tend to close off and lifelong commitments must be made. Arnett calls it “the age 30 deadline。”

　　The issue of whether emerging adulthood is a new stage is being debated most forcefully among scholars, in particular psychologists and sociologists. But its resolution has broader implications. Just look at what happened for teenagers. It took some effort, a century ago, for psychologists to make the case that adolescence was a new developmental stage. Once that happened, social institutions were forced to adapt: education, health care, social services and the law all changed to address the particular needs of 12- to 18-year-olds. An understanding of the developmental profile of adolescence led, for instance, to the creation of junior high schools in the early 1900s, separating seventh and eighth graders from the younger children in what used to be called primary school. And it led to the recognition that teenagers between 14 and 18, even though they were legally minors, were mature enough to make their own choice of legal guardian in the event of their parents’ deaths. If emerging adulthood is an analogous stage, analogous changes are in the wings。

　　But what would it look like to extend some of the special status of adolescents to young people in their 20s? Our uncertainty about this question is reflected in our scattershot approach to markers of adulthood. People can vote at 18, but in some states they don’t age out of foster care until 21. They can join the military at 18, but they can’t drink until 21. They can drive at 16, but they can’t rent a car until 25 without some hefty surcharges. If they are full-time students, the Internal Revenue Service considers them dependents until 24; those without health insurance will soon be able to stay on their parents’ plans even if they’re not in school until age 26, or up to 30 in some states. Parents have no access to their child’s college records if the child is over 18, but parents’ income is taken into account when the child applies for financial aid up to age 24. We seem unable to agree when someone is old enough to take on adult responsibilities. But we’re pretty sure it’s not simply a matter of age。