Rebecca Hecht-Lewis and her husband, Jim, are getting ready to move, uprooting their two young sons and leaving the urban confines of Alexandria for suburban Fairfax County.

The reason: the boys.

"We came here because it was a great place to be single," Hecht-Lewis, 49, a psychologist, said one recent afternoon as she sat in a Del Ray coffee shop helping son Michael, 8, do his homework and waiting for her other son to get out of school.

"But to raise children, we need more space -- at home and outside -- and we are looking for better facilities for our kids," she said, adding that Fairfax schools have an appeal as well. "So we're gone."

Couples with children have been departing the close-in suburbs of Alexandria and Arlington for decades now -- or choosing not to settle there in the first place. In spite of a slight increase in their share of under-18-year-olds because of immigration in the 1990s, both communities have gradually morphed from conventional family-centered suburbs into new-urban enclaves that demographically are more like Manhattan and Chicago than nearby Fairfax, Loudoun and Montgomery counties.

The two communities have the lowest percentages of children in the region. Only about 16 percent of Arlington's 189,000 residents are under 18. Forty years ago, 30 percent were under 18, according to the U.S. Census. In Alexandria (population: 128,283), only 16.8 percent of residents are children, down from 34 percent in 1960.

Compare those figures with suburban Prince William County, where 31 percent of the population is under 18, or Montgomery County, with 26 percent. Even the District, where one in five residents is under 18, has a higher proportion of children than Northern Virginia's inner core. Manhattan matches Arlington with 17 percent.

"These are not your father's suburbs," said Robert E. Lang, director of the Metropolitan Institute at Virginia Tech, who has studied communities like Arlington and Alexandria. "In many respects, they are a world apart from a place like Loudoun, which is just 25 or 30 miles away."

Instead of being home to Ozzies and Harriets, the two communities more closely mirror an episode of "Friends" -- young singles searching for apartments and trendy night life -- or empty nesters looking for less lawn to mow and more convenience closer to the District.

The gradual demographic shift has created new social dynamics and fresh political challenges. PTAs, service clubs and soccer associations are no longer the principal vehicles for community building. Gyms, dance clubs and more than 30 dog parks are more likely gathering places.

Politicians try to balance the sometimes competing interests of their new communities with the needs of their conventional suburban constituents, who still have children and need places for them to play and learn. In Arlington, which will hold a special election for an empty county board seat in March, urbanization has become a political hot potato. Republicans argue that the county, one of the region's first suburbs, has grown far too urban on the long watch of the Democratic board.

And in Alexandria, where the number of singles is growing, managing density is sure to be a major issue in May's council elections.

"We certainly are at a crossroads in many respects," said Alexandria Mayor Kerry J. Donley (D). "We are faced with having to address more complex problems, with the needs of lots of differing constituencies in the balance."

In Alexandria, for instance, some of the hottest civic debates recently have been over where to put dog exercise parks and how to use open space. Last year, townhouse residents argued successfully for a walking path in a waterfront park rather than a marine education facility for youngsters. Plans for a much-needed replacement for T.C. Williams High School that could have spilled into a nearby park also ran into stiff opposition from neighbors who objected to the loss of open space behind their homes.

Arlington has poured millions of dollars into pedestrian walkways, bike paths and expanded Metrobus service in response to increasing requests from the singles community. And the county in some instances has scaled back ambitious plans for playing fields and other school facilities after residents mounted furious opposition.

"It's a continual debate that the county is going to have to struggle with," said Carrie Johnson, a longtime Arlington planning commissioner. "The more we have these differing populations with specific wants and needs, the more we're going to have to pay attention to how we respond as governments."

With childless households have come businesses and entertainers eager to serve them. Constellations of bars and restaurants, largely for singles, along Clarendon and Wilson boulevards in Arlington and Mount Vernon Avenue in Alexandria have changed how people meet socially in formerly sleepy suburban neighborhoods.

On a single stretch in Clarendon, young professionals flock to a Gold's Gym -- good for socializing -- a ballroom dance studio providing lessons and a Fresh Fields that offers groceries as well as classes on how to prepare them.

"The usual mode that helps create community -- schools, PTA, raising children together -- is different than what it was when I was growing up," said veteran Alexandria Council member David G. Speck (D), 57, who grew up in the city. "Now people are bonded by different things, and that means we're a very different place from where we were 40 years ago."

For Karla Kitsfield, 29, the thread that binds social relationships is Wilson Boulevard in Arlington. Kitsfield, a computer consultant, moved from Boston via Sterling, looking for a life that she remembered growing up in the Northeast. But for her, the ultimate attraction -- and why she has stayed for seven years -- is the options: She swing-dances regularly at Clarendon Ballroom's weekly classes. She meets the same group of friends for drinks every week at Iota on Wilson Boulevard. The crowd has become so close they've gone skiing in Vermont and to the Caribbean together.

"The bottom line is that I feel this is a place for us," she said as she left her dance class. "I can't really say if I'll stay forever . . . but you bump into people here at the store, the bike shop, on the trails. . . . We have our own little way of life."

There is a growing interest among some single residents in local politics, although many longtime activists lament that it is difficult to organize isolated singles in apartment buildings. The number of Young Republicans in Alexandria has tripled the past several years, party officials say, and young singles in the Rosslyn and Ballston areas have become involved in several civic associations to advocate for more recreational activities in Arlington.

While there have been clashes between residents over whether to spend public money on facilities for children or quiet walkways for adults, neither Alexandria nor Arlington has seen taxpayers turn away from school needs. Both the city and the county have some of the highest per-pupil spending in the region. Arlington voters have overwhelmingly approved three school bonds for hundreds of millions of dollars over the past decade.

"Arlingtonians understand the value of good schools even if they don't use them -- or never will use them," School Superintendent Robert G. Smith said. "People understand that good schools are good for quality of life, even if they've already raised their children."

But there are still struggles over capital spending. Alexandria, which has built only one new school in a generation, is planning to replace T.C. Williams High School, its only senior high. There has been fierce debate on the rationale for spending $75 million on a school when the high-school population is projected to fall over the next decade.

The quandary has frustrated local officials, who wonder what such battles over resources for children say about the city and its future.

"How we provide services and meet the needs of children and the elderly really defines us as a city," said council member Claire Marie Eberwein (R), a former school board member. "It's something we don't want to lose sight of because we want to ensure that community spirit remains the essence of Alexandria."

"It was a great place to be single," Rebecca Hecht-Lewis, with sons James, left, and Michael, says of Alexandria. Arlington and Alexandria, which have become urban enclaves, have the lowest proportion of children in the region.Fairfax County offers more space and tempting educational offerings for a family, says Rebecca Hecht-Lewis with sons James, left, and Michael, who are leaving Alexandria.