Start with the fact that there are now more than 24 million Americans who are 65 or older. By 2025, their ranks are expected to more than double.

For years, many of our retired citizens have headed for the Sun Belt.

But these days, more empty-nesters are finding themselves able to afford a new style of housing, with appropriate recreational amenities, outside of the Sun Belt.

Increasingly, residents of the Washington region have been able to find retirement communities to their liking here.

Because one-story living is often the housing of choice for older persons, many moved to apartments in past decades. In the 1970's it was likely to be a garden or high-rise condominium -- a trend that will continue in the 1980's. Many of the area's new, fairly posh condominiums are purchased by people who are still working but plan to retire in a few years.

Because some like to continue living in houses, developers here in coming years are likely to recognize that lucrative market and will open special adults-only sections in their communities. Some adult apartment buildings may be constructed if developers perceive that they can make money from them.

This region has one seasoned adult community -- Rossmoor at Norbeck -- and two newcomers, Crestwood Village, southeast of Frederick, Md., and Heritage Harbor at Annapolis. All three felt the crunch of the winter's high interest rates and downturn in buying.

During the past year, 50 houses have been sold at Crestwood Village, said Herbert Wishnick, the operations's executive vice president. It was "not what we hoped for, but not bad in the tough market," he observed. "We are now seeing more people coming out and we have new, larger, more expensive houses that we think will appeal to this market."

The firm previously had built more than 12,000 adult community homes in New Jersey. David Wolff, Crestwood's marketing director, said the company had "learned that middle-class people nearing retirement are looking for a new style of living, probably a bit better and certainly more carefree than they had when they were raising children."

Rod Garshag, marketing director for U.S. Home Corp., which builds for the 45-and-over market in its Heritage Harbor community near Annapolis, agreed with Wolff that many purchasers are attracted by recreational amenities. Heritage Harbor had 120 sales during its first year of operation, he said.

Heritage has lost some sales to persons "who have fallen in love with the one-story homes but have regarded the commute to Washington as too far," he noted.

At 15-year-old Rossmoor, which now has more than 4,000 residents, "We no longer build single houses, but the sales have been good for the new apartments," said marketing director Ron Santos. "There are only three in unsold inventory and we plan to start more this summer. There's land to build at least 2,000 more dwellings here."

Meanwhile, there are recurring reports that Californian Ross Cortesi, the original developer, plans to sell Rossmoor soon, but the rumor could not be confirmed.