Shannon Rea's job as a part-time bartender in Brooklyn gives her a close-up look at the risky behavior of people in their 20s.

Some end a night of drinking with hookups. Some take rides from the slightly inebriated. Others try to drive when they shouldn't. (She sobers them up, takes their keys and finds them rides.)

"I think the early 20s are the new teenage years," says Rea, 26, a college student studying to be a history teacher. "There are no parents telling them, 'You can't do this.' It's pretty much a free-for-all."

The 20s always have been prime time for risky behavior, from binge drinking and unprotected sex to dabbling in drugs and driving too fast. But new brain research suggests young adults may have less control over these impulses: Neurological areas that regulate impulse and emotions are not fully developed until about the mid-20s, findings show.

And recent demographic trends don't help: Young people today are delaying settling down into careers and marriage, both of which tend to reduce risky behaviors, sociologists, psychologists and historians say.

"We are so obsessed about the dangers of adolescence, we have all sorts of mechanisms to protect kids from disaster, but we don't have that for young people in their 20s," says Steven Mintz of the University of Houston, who is writing a book on the history of adulthood.

American society has taken great pains to prevent adolescents from the perils of foolhardy behavior, from zero-tolerance alcohol and drug programs to graduated drivers' licenses to city curfews for youths, Mintz says.

If teens make mistakes, they often get a second chance. But in their 20s, society is less forgiving.

"It's the 20s that are risky," he says.

Statistics tell the story

Recent statistics back this up: 56% of births among women ages 20-24 were to unwed mothers, according to preliminary 2005 data from the National Center for Health Statistics; for ages 25-29, it was 29%.

Violent crimes — from homicide to rape, robbery and assault — are highest among young adults, according to data from the federal Bureau of Justice Statistics and the FBI.

The 20s also is the time of heaviest drinking. One measure, the University of Michigan's 2005 Monitoring the Future study, shows that the highest percentages of those having five or more drinks in a row at least once in a two-week period were those in their 20s. Bingeing was reported by 40.4% of ages 21 and 22, 39.2% of ages 23 and 24, and 37.7% of ages 25 and 26.

When young celebrities such as Paris Hilton and Lindsay Lohan abuse drugs or alcohol or drive under the influence and get caught, the big question seems to be: Why?

Scott Stanley, a research professor in psychology at the University of Denver, says many major life decisions that used to be fairly settled are now "up for grabs through their 20s" — from education and career to sexual relationships and partner choices.

This "extended adolescence" increasingly is being acknowledged by those who have focused on teens. For example, in May, the National Campaign to Prevent Teen Pregnancy announced an expansion of its mission to include those in their 20s and 30s.

At least 35 states have taken some steps to extend foster care services to those ages 19-23; 18 has been the point for "aging out" of the foster care system.

Most states consider 18 the time when young people are legally held responsible for their actions. But Jeffrey Arnett, a research professor in psychology at Clark University in Worcester, Mass., says many young people just don't believe they're going to suffer the consequences. He calls it an "optimistic bias."

Arnett says young celebrities in particular so often have been overindulged as children and teens that they don't have the self-discipline to play by the rules.

Britney Spears' risky behavior hasn't just affected her. Last year, the singer, now 25, was photographed driving with her infant son sitting on her lap behind the wheel rather than in a car seat.

But real social and economic factors today do add to the pressures facing young adults:

•A study of long-term wealth trends released last week by the University of Michigan's Institute for Social Research found the median household net worth of people in their 20s fell by nearly 30% from 2003 to 2005. The data support the notion that it is more difficult for younger people today to establish themselves financially, says study director Frank Stafford.

•Finishing college often takes longer than four years; some also stay in school longer because in an increasingly competitive job market, they seek to get a leg up with advanced degrees.

•Some experts, including Mintz, say the military draft (created in 1940 and eliminated in 1973) forced young men to mature and at the same time helped keep them away from troublesome behavior. The military used to be a pretty standard part of growing up for men in decades past, but it's a route for fewer young people today — "it's a risky proposition these days," he says.

Adds bartender Rea: "When you're 15 or 16, you're expected to go out and experiment and break curfew and do something ridiculous. You're immature at that point. Then at 18 or 19, something is supposed to snap on in your head, and you're supposed to grow up."

The biology of the brain

Over the past several years, brain studies by researchers around the country, including at the National Institutes of Health, University of Pittsburgh, Harvard Medical School and Temple University, have found that the area that controls impulses takes longer to mature than previously thought.

Greater demands have made the 20s a difficult period, says Frank Furstenberg Jr., a sociologist at the University of Pennsylvania in Philadelphia whose work has focused on the transition to adulthood.

"It is not well known that this age group does have these elevated problems," he says. "Adolescence has captured so much of the attention of American policymakers."

He says it's even more of a challenge for an estimated 15% to 25% who by early adulthood are "seriously off track."

They include those who aren't in school, don't have jobs or may have little prospect of finding a job due to lack of skills. Others have serious drug or alcohol problems. Some are in jail or otherwise involved in the criminal justice system.

"It can be a risky stage for rich kids because they have the money to get in trouble and for poor kids because trouble finds them in the neighborhood," says Michael Rosenfeld, a social demographer at Stanford University. "There are plenty of ways in which kids from families with the financial means avoid the long-term repercussions of youthful indiscretion."

Partners, careers stabilize

John Laub, a University of Maryland criminology professor, and Robert Sampson, a Harvard sociologist, have studied the life course of crime for about 20 years. Sampson says the peak age for many crimes has shifted older, and although juveniles are committing offenses at younger ages, the duration of criminal activity lasts longer.

By analyzing interviews with 500 men, all of them former juvenile delinquents, and follow-up interviews with 52 men, the pair also found, on average, a 35% less chance of a crime being committed during a period of marriage.

"We find that marriage 'civilizes' men — reducing crime and antisocial behavior on average by a large amount," Sampson says.

The average age at first marriage, according to the latest Census data, is 25 for women and 27 for men. Experts say singles overall are more likely than married people to take risks because they have the freedom to do what they want without having to answer to anyone.

A few years do seem to make a difference. All sorts of research suggests that by the late 20s, risky behavior drops among young adults, largely because that's when they pair up and begin to settle down with a career and a partner.

Furstenberg says social relationships tend to constrain behavior. "People stop doing things they did when they were younger."

Beth Kerber, 26, of St. Louis, believes she has definitely matured, although she says she "can't imagine" marriage right now.

"I've seen a huge amount of growth in myself from age 22 to 25," says Kerber, who just finished graduate school and is working as a hospital speech pathologist.

"When I go out now, it's completely different than when I was 21 or 22. You are much more experienced with drinking and realize it's not that great to black out or not remember the night. When you're young, it's 'Let's have fun.' "