People in their 20s and early 30s now make up almost a third of the District’s population, new census figures show, and were responsible for almost all of the city’s growth in the past 10 years.

The surge of young people reflects the city’s growing allure to college graduates, who come to take advantage of the strong job market and help transform neighborhoods into lively centers of condominiums, clubs and coffee shops.

The District also experienced an increase in adults in their late 50s and early 60s during the past decade. At the same time, the number of children younger than 15 dropped by a fifth.

But the biggest change was the spike in the number of young adults.

About 190,000 of the city’s 600,000 residents are between the ages of 20 and 34, a 23 percent jump. The 35,000 additional people in that age group fueled the city’s entire population gain between 2000 and 2010.

They did not settle evenly around the city. The increase was steepest in the wards that encompass Capitol Hill, the Northeast, downtown, Shaw and Logan Circle. The growth among young adults was negligible in the city’s poorest neighborhoods and in its most expensive. The census figures could not be broken down by race.

Demographers and urban experts said the census numbers confirm what is readily evident in Washington’s crowded sidewalk cafes and bike lanes.

“This reflects the resurgence of the city over the decade,” said Alice M. Rivlin, who was chairwoman of the D.C. Financial Control Board. “It’s clearly become a more lively place, with more restaurants, more theaters, more things young people and empty nesters like to do in cities.”

People such as Yuuke Shinomiya.

When he came to the District six years ago from Ohio to take an internship on the Hill, most of the people he arrived with were gone after two years. He intended to follow but stayed for a job with a medical association and became entranced by the city.

“I was just going to be here for six months,” said Shinomiya, 28, who works as a lobbyist for an insurance company. He lives in a Glover Park apartment, practices yoga and bar-hops not just in Adams Morgan and Dupont Circle, but lately also around H Street, Penn Quarter and Logan Circle. “It’s just steadily gotten a lot more hip. A lot of my friends who left for grad school have come back.”

‘Spots cropping up’

That humming singles vibe has caught the attention of David Helfrich, 29, who is just finishing law school at Howard University. Originally from New Hampshire, Helfrich said he hadn’t planned on settling here but is looking for a local job in intellectual property law.

“I’d like to stay. I kind of fell in love with the city,” Helfrich said Wednesday while shopping at the Whole Foods market on P Street in Northwest. His top D.C. draws: the international tone of the city, the robust legal scene and the clubs. Helfrich is a frequent partier at Josephine and Recess in Northwest and Smith Commons on H Street.

“There are spots cropping up all over to cater to people like me,” he said. “It’s gotten to be the perfect mix of professional life and night life. That’s hard to find anywhere.”

One of the ‘cool cities’

Their impressions underscore the District’s newfound cachet. A city once renowned as a mecca for workaholics is starting to be thought of as a place that’s fun.

William H. Frey, a demographer with the Brookings Institution, said the District is following the same pattern seen in Boston and other places considered to be “cool cities.”

“Young professionals [are] inhabiting the city while boomers have left to go to the suburbs,” he said.

Ed Lazere of the DC Fiscal Policy Institute said the growth in jobs requiring a college education and the city’s investment in Metro and other services are a draw.

“People are moving to the Washington area, and then deciding they’d love to live in the District because it’s an exciting place to live,” he said.

The ranks of people 55 to 64 are rising, too. They went up from 56,000 a decade ago to 64,000 today and account for 11 percent of the city’s population.

But it is largely young adults who have been targeted by many developments the District has pursued.

“In the wards that have seen the most population growth, part of that growth has been from younger professionals moving into the city and into all the new housing created in those areas, probably specifically for them,” said Peter Tatian, a researcher at the Urban Institute’s Center on Metropolitan Housing and Communities. “A lot of singles and childless couples make up that population.”

It remains to be seen whether they will stay as they age, marry and have children. The census shows that the number of children younger than 5 remained stable over the decade, but the number of children ages 5 to 14 fell 20 percent, from 65,000 to 51,000.

Some urban planners consider households without children a financial plus because families with school-age children cost cities more in services than they pay in taxes. Others say cities with few children are poorer for it.

“People with children are good for cities,” Frey said. “They invigorate neighborhoods. They’re more involved in community activities, making sure the libraries work well and the parks are working well. Families with children add a sense of social interaction to a community.

“One drawback of the District’s growth is that it’s been in people who are younger and older, but not in the middle.”

That’s the pattern that Orlando Fuentes sees in his work managing condominium buildings. His typical resident is an unmarried young professional who stays a few years. Once married, they don’t stay long.

“It still feels basically like a singles city,” said Fuentes, 37, who was riding a neon blue Schwinn on the new bike lane along 15th Street NW. He moved from Hyattsville in 1994 because of the District’s reputation as a gay-friendly city and stays, in part, he said, because it has become such a bike-friendly one.

Shinomiya, the insurance lobbyist, said he envisions staying in the city even as a parent.

“At this point, all of my friends are here and all of my professional network,” he said. “Practically speaking, I think I will be in Washington for a long time.”