Steve and Aneliz Sipe care for their 9-month-old son, Will, in their Arlington condominium earlier this month. They recently moved to a single-family house in Fairfax County. (Brittany Greeson/The Washington Post)

The condominiums on the cul-de-sac in Fairway Village have sold steadily over the past eight years, transforming a solidly middle-aged enclave into a community of starter homes for the oldest members of Arlington’s millennial boom.

And now the young adults who have enlivened and enriched this urbanizing suburb just across the Potomac River from the District are starting a new generation; there have been 10 infants born to the 37 households on the cul-de-sac since 2008, five in the past year.

With the babies come questions, for Arlington and for other millennial-dependent cities and suburbs: Will the newcomers stay as they age out of young adulthood? Can a place that lured this large generation with walkable neighborhoods and convenient transit now offer the classroom space and affordable housing that they say they will need to remain?

Steve and Aneliz Sipe bought one of Fairway Village’s townhouse-style condos in 2012. They loved their short commutes and the easy walk to restaurants on Columbia Pike. But they moved recently to a single-family house in Fairfax County that offers more room for their infant son, a yard and proximity to Aneliz’s family. Their two-bedroom condo sold 10 days after they put it on the market to a couple that Steve described as “us, three years ago.”

Arlington has always drawn lots of young people, but the millennial generation — those born roughly between 1980 and 2000 — is bigger than those that have come before. It is the most populous cohort since the post-World War II baby boom.

So many young adults have moved to Arlington since 2000 that the median age in the county has dropped by more than two years, from 35.9 to 33.7.

The latest census numbers show that 25- to 34-year-olds make up about 27 percent of Arlington’s population, up from about 19 percent in 2007 — a greater share than any other city or county in the region. In the spring, a 30-year-old won one of two Democratic nominations to the Arlington County Board, where the average age of the members is nudging 61.

Keeping as many of these highly educated and tech-savvy residents as possible is a critical factor, experts say, if the county wants to attract employers and build its tax base. If the county succeeds, it could become a model for such millennial destination cities as Austin, Denver and Portland, Ore., that have seen a similar population bulge over the past decade.

To that end, the county is considering a study of why residents move in or leave the county, and political leaders are talking about new housing options, including relaxing the rules about sharing single-family homes.

“Arlington is again on the cutting edge,” said Steven Pedigo, director of New York University’s Initiative for Creativity and Innovation in Cities. “Nobody’s doing it yet on a large scale. . . . The county either has to find a policy to keep the millennials or say, ‘We’ll accept the cycle of people moving in and out.’ ”

Pie for one

The millennials have fueled a yoga boom in Arlington and persuaded the county to plow snow from bike lanes. Luxury apartment buildings lured young residents with wine and bicycle storage lockers, concierge desks and on-site gyms.

Aneliz and Steve Sipe moved to a single-family home to gain more room for their infant son Will, a yard and proximity to Aneliz’s family. (Brittany Greeson/The Washington Post)

In response to customer requests, the Whole Foods in Clarendon has started opening earlier and closing later.

It launched a Peruvian chicken take-out window, which doesn’t even require that customers enter the store, and turned its balcony cafe into a pub with craft and specialty beers and a four-hour happy hour — a big hit with its customers, at least 40 percent of whom are millenials, marketing team leader Jackie Zovko said.

The bakery sells a lot of mini-pies and slices of desserts rather than whole cakes. During the holidays, Zovko said, the store leads all Whole Foods locations in the Mid-Atlantic in sales of tabletop Christmas trees: “We can’t keep enough in stock.”

The lively sidewalk dining along Campbell Avenue in the county’s Shirlington neighborhood “makes us a little more of a community,” said Lauren Yoder, 30, who was drinking wine and sampling cheese with her husband and a friend on a recent night. “We’ve been down here twice today already — I brought our dog this morning.”

The county’s public library system has built a robust series of programs that cater to young professionals, sponsoring trivia nights in local bars, book clubs in restaurants, and adult recess and game nights at the Central Library. Demand at the 15 recreation centers has prompted them to extend their hours, too, with some staying open from 6 a.m. to 10 p.m.

“We love the accessibility to the city and walking on the trails. We probably go to the park four or five times a week,” said Anna Duran, 35, a mother of three who was taking her daughter to an American Girl reading event at the library.

Changing needs

Arlington’s Virginia Hospital Center has seen a steady increase in births over the past few years. The number of babies born to women in their 20s rose from 1,360 to 1,640 between 2011 and 2014, a hospital spokeswoman said. The number of babies born to women in their 30s jumped by almost 800 in the same period, to 3,724.

As millennials have children, their priorities expand to include good schools and family-friendly neighborhoods. A condo or townhouse can suddenly seem too small, and a bigger place in the same neighborhood may be too pricey.

Six in 10 surveyed Arlington residents between the ages of 25 to 34 told the county’s affordable-housing study group last year that it’s somewhat likely or very likely that they will move out of the county within five years because of housing costs.

“I love the Clarendon neighborhoods, but who can afford it?” said Heather Shepherd, 30, a neighbor of the Sipes, who is pregnant with their first child. “That’s the frustrating part.”

Three-quarters of the millennials in Arlington are renters, county data shows, and one-quarter of them spend more than 35 percent of their income on housing, far above the recommended limit of 30 percent.

New apartments in high-rise complexes in the Rosslyn-Ballston corridor, which includes Clarendon, rent for an average of $2,351 per month, according to the real estate firm Delta Associates, on par with pricey D.C. neighborhoods. The median price of an Arlington condo, townhouse or single-family home is up 7.6 percent from last year, to $570,000.

“It’s hard to have long-term plans with [the cost of] real estate the way it is,” said Liz Belfield, 29, who lives in the Court House neighborhood.

The county’s ability to help young professionals with housing costs is limited. Government subsidies are reserved for much poorer individuals and families, who also have scarce options in Arlington.

But County Board members and candidates for the two open seats are beginning to kick around ideas such as revising rules that govern sharing single-family homes, or encouraging more micro-unit apartments where residents could also share work space.

Arlington’s public schools are well-rated nationally. But enrollment has grown 19 percent in the past five years, and school officials say they expect a 2.7 percent annual growth rate for each of the next 10 years — the equivalent of one new elementary school each year, in a county where open land for building is scarce. Class sizes have increased, and so has the number of classes being held in trailers.

Katie Roth, 34, a South Arlington mother of two, said her older son is thriving at Barcroft Elementary even though the school has portable classrooms and his kindergarten class was split in two last year because it was so big.

She and her husband may look at houses elsewhere in the county, where the schools are less crowded. But they want to stay in Arlington, because of the diversity and the relatively easy commute to her job as a teacher in the District.

Overcrowding is also a worry for Lauren Landrisan, 28, even though her daughter is 18 months old. “We may consider private schools, but we will look into the public schools,” said Landrisan, who lives in Shirlington. “We’re trying to figure out what to do.”

Preventing an exodus

Historically, young adults have moved into Arlington starting around age 20 and left for other suburbs in large numbers once they reach their early 30s.

Federal employee Elsie Oldaker, who was among the hordes of young people pouring from the Court House Metro station one recent night, said that at 30, she sometimes feels “like I’m getting too old for this area.”

But the millennial generation has confounded other past trends, including embracing public transit in ways its predecessors did not.

Elizabeth Hardy, Arlington’s planner and demographer, said the county is planning a study of whether the out-migration of Arlington’s 30-somethings will continue. “Once we have an understanding of the factors that cause people to come or go, we could address how to keep them here,” she said.

And since the millennial boom extends to young people born as late as 2000, Hardy says she expects the demographic to remain important to Arlington for the next several years.

“We still have a large group who could move in as they age, even as [older millennials] move out,” she said.

Back in Fairway Village, Heather Shepherd and her husband know three couples who have moved to Annandale, not including the Sipes. For now, the Shepherds do not intend to follow suit. They like the convenience of Arlington, close to city attractions and the D.C. law firm where Shepherd works in an administrative job. The schools seem good, she said, and Fairfax County seems too far away.

“Our immediate plan is to stay.”