ADOLESCENCE IS A PERIOD of no-longer and not-yet - a time of being no longer a child but not yet an adult.
Once, in a simpler era, this stage of life - the subject of a million jokes - was generally defined as beginning around age 12 and ending in one's early 20s. A 21st birthday was cause for relief and rejoicing: "Whew!" parents might think. "We made it through."
Now those old boundaries are changing. Adolescence stretches longer and longer. Puberty, marking the end of childhood, begins earlier. And the signposts of adulthood - finishing school, starting a job, leaving home, getting married - are occurring later. Young people must spend more time in school to succeed in an information society.
"The ladders required to get to adulthood are lengthening," says Reed Larson, coeditor of "Adolescents' Preparation for the Future" and one of three researchers studying the globalization of adolescence.
Curious contradictions can result. Precocious 10-year-old girls pose as provocative Britney Spears wannabes. ("Look, Mom, I'm sophisticated.") At the same time, 23-year-old college grads, unable to afford their own apartment, boomerang back to the parental nest. ("Hey Mom, where's my clean laundry?")
No wonder some young people don't think of themselves as fully adult until their mid-20s. Researchers call this ambiguous period "emerging adulthood." All grown up, sort of, with no place to go.
Equally significant, adolescence - once recognized mostly in developed nations - is becoming a defined stage of life everywhere. In rural India, Mr. Larson says, the beginning of adulthood is shifting from age 12 to 16. In postindustrial societies such as the US, it is rising from age 22 to 26 or later.
The World Wide Web is also changing young lives. It provides new social spaces for getting acquainted, at least virtually. As one example, Arab boys and girls, prohibited from face-to-face meetings, now use the Internet to talk with one another.
On the downside, young people who lack access to computers and training - the great digital divide - face limited opportunities and diminished life chances.
Within the family, the need for multiple incomes keeps young people and parents apart for longer periods during the day. Peer groups exert more influence, leaving teens vulnerable to earlier sexual activity.
Global demographics are also changing adolescent roles. In the developed world, for the first time, the old are outnumbering the young. "Adolescents will need to be prepared to support ever-increasing numbers of elderly people as the century progresses," says researcher Jeylan Mortimer, one of Larson's coeditors.
In developing countries, the reverse is true. There, the growing number of young people is outstripping the ability of nations to educate them and prepare them for work and adulthood.
The key question, Larson says, is this: Is adolescence changing in ways that positively affect young people, or are they ill-prepared for their adult responsibilities?
Although the demands on adolescents are increasing, opportunities for them are expanding, too, he says. In other good news, young people in most parts of the world report that they are optimistic about their lives.
Adults everywhere must reinforce and strengthen that optimism, the researchers emphasize. Yet they caution that adults often fail to recognize the significant contributions younger generations of volunteers make to their communities around the world.
Larson puts it this way: "If we just look at youth as problems, we get more problems. If we look at youth as resources to be developed, then it can make a large difference, not only for those youth but for society as a whole."