America's suburbs grew up along with the baby boomers, and now that the group is approaching retirement age, most communities designed for them are being built in the suburbs.

Nine of the top 10 counties in the nation for recent active-adult home construction are in the suburbs, showing the tendency of many older buyers to move just outside metropolitan areas. Nine of the counties are in the Southwest and Colorado. One is in Florida.

"About three-quarters of the active-adult communities that are built are in close-in suburbs or outer suburbs, although downtown areas are steadily gaining popularity," said Bonnie Solomon, chairwoman of the National Association of Home Builders Seniors Housing Council and a vice president of several retirement communities in St. Louis, Kansas City, Mo., and Atlanta.

"Builders are simply responding to the market demand," she said.

The term baby boomer includes people born from 1946 to 1964, meaning those who are now 40 to 58 years old. There are 85 million baby boomers in the United States, according to estimates.

Many boomers are moving to active-adult communities before they retire, choosing to change addresses as soon as their children have moved out. Dave Schreiner, vice president of Pulte Homes Inc., said many boomer buyers were beginning "their 'someday stage,' before they quit working."

From 20 to 50 percent of those who buy houses in age-qualified active-adult developments by Pulte's Del Webb unit do not plan to retire when they move in, he said. Those percentages depend largely on the availability of full-time or part-time employment nearby.

About 80 percent of those responding to an AARP poll said they planned to work after age 65, whether full time or part time, said Constance Swank, the organization's research director.

Retirement today can span 40 active years. Financial concerns seem to weigh heavily on 55-year-olds as they approach retirement age, Swank said.

Boomers are still uncertain how much money they will need to finance retirement. Of those surveyed by AARP who ventured a guess, fewer than half said $500,000 or more, while 17 percent suggested they would need at least $1 million.

William Frey, a professor of demographic studies at the State University of New York at Albany, said baby boomers were making the active-adult market younger.

Even if boomers move into such housing before turning 55, they do not dramatically alter their lives. They continue to work by consulting or telecommuting. They also tend to remain in the areas in which they had been living.

"The difference between the baby boomers and their parents is that the parents were willing to defer gratification until they reached retirement age, while the baby boomers want it all now," Frey said.

Although the catalyst for boomers to move to active-adult developments appears to be children leaving home, this newfound independence often does not last long. Almost 25 percent of boomers responding to surveys say they think their children will move back in with them.

Such "boomerang" children appear to be a growing social phenomenon. More than 25 percent of Americans ages 18 to 34 live with parents, the Census Bureau said. Among 18- to 24-year-olds, 56 percent of men and 43 percent of women live with one or both parents.

A Del Webb survey of its baby boomer buyers found that 65 percent would be happy if their grown children moved back in, and that almost 25 percent would feel obligated to help. Fathers were more eager than mothers to find other living arrangements for their adult children, the survey found.