WASHINGTON — In an Alexandria, Va., strip mall, Erin Rexroth watched her 21-month-old daughter, Haley, straddle and bounce on a plastic alligator one recent day at the My Gym. Rexroth, 27, stood apart. On the playroom's other side, older parents chatted with a nanny and a grandmother next to a plastic basketball hoop.

"I don't feel quite mentally in mom-world," Rexroth said, glancing at the others. "But you do whatever makes your kids happy. At least the gym doesn't play annoying kid music all the time."

Rexroth, a former congressional aide, and her husband, Philip, 27, who works for the Department of Homeland Security, are defying the norm for their class and age group: They are raising a child. The majority of college graduates in their 20s in metropolitan regions postpone having kids until at least their 30s or never have any, according to recent demographic research.

Like anyone who strays from the generational pack, college-educated parents in their 20s often face questions about friendships, careers and their place in life. Although rearing children invigorates them like a high-profile job, these parents sometimes say they feel like guinea pigs among childless peers. They wonder whether it's possible to befriend older parents. Some say they feel isolated from friends, those who don't change diapers or deal with sleep deprivation.

On her drive home, Rexroth lamented the last-minute cancellation of a lunch with old Capitol Hill friends. "We would have talked politics, what they're doing about work and reminisced about crazy days and pranks," she said. "I was disappointed."

Demographic data obtained by The Washington Post indicate that in metro areas nationwide, including cities and suburbs, 13 percent of men and 31 percent of women ages 25 to 29 with four-year college degrees have had children, according to an analysis of 2000-06 social survey data from the University of Chicago's National Opinion Research Center. By contrast, 49 percent of men and 62 percent of women in that age group with less education have had children, according to the analysis by University of Maryland sociologist Steve Martin.

New data from the National Center for Health Statistics also show that college-educated mothers are usually about 30 when they deliver their first child.

"This is very significant data. It's giving numbers to a trend people have been only inferring," said Stephanie Coontz, director of research at the Council on Contemporary Families. The data, she said, show that "there is this increasing divergence of highly educated women and less-educated women."

College graduates who have children when they are young say they are irritated when they hear suggestions that they are giving up on careers. They also contend with feeling alienated even in casual moments, when, they say, childless friends ask awkward questions such as: Was it planned?

"If we already had a child or were a bit older, no one would have asked that," said Brett Libresco, 29, a research analyst at the World Bank. He and his wife, Liz Johnson, 29, a part-time program analyst, were the first among of their local friends and colleagues of their age to have children. "Sometimes, what you think is behind that question is, 'It must be an accident,' because none of our friends are doing this."

Many delay children because they have delayed marriage. From 1950 to 2004, the median age of first marriage rose from 20 to 26, according to a new book, "The Price of Independence: The Economics of Early Adulthood," co-edited by Sheldon Danziger, a University of Michigan public policy professor. Another factor in delaying having children, Danziger said, is that women and men who live together without being married are more socially accepted than ever. "Cohabitation is a trial marriage," he said, "and a substantial number of cohabitations don't become marriages."

Danziger said many people who seek high-profile jobs in metro regions face a long educational haul during their 20s. They might finish college, work for a few years, go to graduate school and then try to cash in on expensive degrees in their late 20s.

Young careerists who begin child-rearing often juggle a love of their new life (including the bottles of zinc oxide anti-rash cream) with nostalgia for happy hours at places such as nightclubs with a young, upscale clientele. They are sometimes uneasy among older-looking parents and worry that they might be considered too young or unsophisticated to have children.

"I still don't really feel like I am an adult. When I walk up to the day care with Emmett, I always feel like they are looking at me and thinking, 'Oh, she's the babysitter dropping off the kid, or whatever,' " said Amy Elliott, 28, a George Washington University law student whose son attends the Broadcasters' Child Development Center in Washington. "I look a lot younger, and I am not dressed professionally like they are."

Amir Eftekhari, 32, a manager for a consulting company, and his wife, Luma, 31, a former systems analyst, had the first of their two children when she was 29. Eftekhari said he senses friction with older parents at soccer games or at a playroom. Older mothers, he said, sometimes seem envious about the Fairfax, Va., couple's head start on child-rearing.

He also said it can be hard to connect with other soccer dads. "One of them was wearing, like, a Rolling Stones type of sweat shirt. I like the Rolling Stones, but I didn't grow up with them. I can't say, 'Hey, let's go grab a cup of coffee,' " he said. Other parents, he said, tell them, " 'Honey, you need to take time for yourself.' They talk about leaving the kids with your grandmother and going to Cancun. We would never do that."

Some couples take the plunge when they're young to reduce the odds of miscarriage and developmental problems. After 40, women have a 50 percent chance of miscarriage, said Arthur Becker, a senior partner at the Obstetrical and Gynecological Group in Bethesda, Md., and Washington. Other moms said an early start has given them a career edge.

"By the time I'm at a point in my career where I am going to be making partner, my kids are going to be old enough to be playing on their own and sleeping on their own," said Erin Foley Lewis, 28, an associate at the law firm Cadwalader, Wickersham & Taft who recently had twins. "If I had waited until 33 to have children, I'd have newborns at the time I would be up for partner."

Others who have take time off are easily irritated at the suggestion that they have "opted out," a controversial expression for women who leave high-flying careers to raise children. People in their 20s said they are too young to feel they cannot resume their professions and excel.

"The expression sounds permanent, but a lot of parents want to do a little bit of both, moderated to some degree," said Liz Johnson, who was given a one-year leave from a consulting job while she raises her 7-month-old, James. She also works 10 hours a week at an organization that gives state governments federal funding information.

"Am I going to be able to do the things I want to be able to accomplish, given that 90 percent of my time goes to the baby?" she asked. "Having a baby is definitely a risk. You risk losing a devotion to yourself, to other things like a cause. I have one friend who goes in the evenings to get his PhD in social policy. ... I don't know if I'll ever have time to do that."

As Johnson and Libresco changed their son's diapers one morning at their home in Alexandria, they talked about how their new schedule for sleep, which doesn't really exist anymore, astounds their friends. "I had lunch with my college friend yesterday, who was asking what it was all like. I said I got up at 5 a.m.," Libresco said. "She was like, 'I can't believe you're doing this.' She said she had been thinking of adopting a toddler. I wasn't sure if she was joking."

Johnson said she laughs when she tries scheduling Sunday brunches with childless friends in Washington. "Ten o'clock is an early brunch to them, but we've been up for four hours by then," she said.

When friends turn down invitations to hang out, the rejection can prompt a flash of envy. "Two friends of mine are married. They travel constantly. One is finishing law school; the other is getting a master's in public policy," Johnson said. "We invite them over, and we get these responses like: 'So sorry. We can't come. We'll be in the Galapagos for the week.' They later sent pictures. We were like, 'Wow, that's amazing.' "

Talk at home might revolve around the frequency of eating solids and replenishing baby clothes, but the couple said parenthood is giving them a new level of ambition that is sophisticated and rejuvenating. "When you arrange an environment and provide guidance and see that it actually happens, all the things you're working on, it's this feeling of joint accomplishment between me and Liz," Libresco said. "That's this bliss."

But sometimes that bliss gets upended on a moment's notice. "Our nanny quit yesterday," Libresco said one day this month. "We haven't spoken with her. She just left a note on the door."