The Ballston neighborhood is home to a mix of high-rise apartments, condos, townhouses and single family homes. (Amanda Voisard/For the Washington Post)

Arlington County, to which thousands of affluent, educated millennials have flocked for its ­inside-the-Beltway nightlife, urban lifestyle and once-reasonable apartment costs, is becoming unaffordable, according to a county-sponsored poll and a separate analysis.

The rising cost of shelter in Arlington affects far more people than just those between the ages of 25 and 34, with 2 in 5 residents saying they are likely to move out of the county in the next five years because of housing costs.

Particularly at risk are renters whose four-person household income is below $64,500 per year, families with children, seniors and people with physical, mental or emotional disabilities.

“The county’s rental stock appears to be serving fairly well both families and single-person households that have incomes above 80 percent of the area median income” — about $60,000 for a single household or $86,000 for a household of four — “and the home ownership market serves well only higher-income households,” said an analysis by Lisa Sturtevant from the National Housing Conference and Center for Housing Policy and Jeannette Chapman of George Mason University’s Center for Regional Analysis.

They said that the current stock, however, is far below what would be needed to meet the demand of renters — particularly families — with incomes below 60 percent of area median incomes.

Elyse, 11, left, and Ingrid, 8, Soracco practice their soccer skills in Arlington’s Rock Spring neighborhood on Monday. (Photo by Amanda Voisard/For the Washington Post) (Amanda Voisard/For the Washington Post)

Most of the homes for sale in 2013 cost $450,000 or higher, considered affordable only to those with incomes above $100,000. The pair’s findings are from the latest version of a three-year study of affordable housing in Arlington and will be presented for community discussion at 6:30 p.m. Monday at ­Washington-Lee High School. A year ago, the first portion of the study showed that an increase in Arlington rents had significantly outpaced the rise in incomes.

This year, a telephone poll of 1,744 adults in Arlington revealed the fear, particularly among renters, of being forced out of their housing in the next five years because of rent increases. They have little hope of finding a replacement home in the county within their budget. The poll, based on a random sampling with a margin of error of 2.3 percent, was taken between April 23 and June 12.

According to the poll, most renters would like to buy a home in the county, but fewer than half think they will be able to afford it. A significant minority — 37 percent — say their current housing costs are unaffordable, in part because they are spending more than 30 percent of their gross income on housing. One in 5 spend more than half of their income on housing.

Arlington’s home prices rose by 140 percent and its rents jumped 91 percent in the past decade, the study shows. Average monthly rents in 2012 ranged from $1,422 for an efficiency to $2,782 for a three-bedroom apartment, and costs have risen since then.

The poll showed that residents want the county to do more about affordable housing. Arlington is spending more than $57 million for ­affordable-housing assistance in a variety of forms, and it negotiates with developers to set aside some of the new construction for affordable units.

Ninety-three percent of those polled want housing assistance for persons with disabilities; 92 percent want the county to help seniors age in place; 91 percent support providing homeless shelters; and 90 percent agree that taxpayers should help low- and ­moderate-income families with children in local schools to remain in the county.

The millennial generation, whose migration to Arlington began in earnest in 2007, now makes up 39 percent of the county’s population, according to Realty­Trac, which collects and publishes real estate data. As these residents age and form families, the cost of housing may weigh more heavily on them as it does for the older residents of this dense urban place. RealtyTrac, in a report this week, noted that the older baby boomers are increasingly leaving Arlington, reporting a drop of 21 percent between 2007 and 2013. They now make up just a fifth of the population.