Retirement in an urban ethnic neighborhood a few decades ago meant Grandma and Grandpa sold their duplex, moved into a basement in-law apartment with their grown children and became built-in baby-sitters for grandkids and a back-yard vegetable garden.

But as the 1970s come to a close, things have changed dramatically for those Americans entering their twilight years. Senior citizens no longer are sitting in their rockers waiting to grow older, according to housing analysts who've studied the retiree market.

Instead, more people are taking what psychologists describe as an adventuresome ride through "life style passages" as they grow older. And their housing preferences shift gradually year by year as they coast through retirement.

Those in the business of housing the nation's retirees are beginning to discover that this is one of the fastest-growing segments of the home market.

There were 14 million heads of households over age 65 in 1975, U.S. Census figures show. Projections indicate there will be 16 million senior citizen households by 1980 and 19.5 million in 1990.

While some senior citizens are heading for the tranquility of maintenance-free, recreation-oriented housing developments in the distant suburbs, a big slice of the retiree market is seeking a warmer climate after the recent bitter winters in the northeastern quarter of the nation. Seniors are moving in greater numbers to Florida, California and Arizona.

In addition to better weather, retirees are looking for a completely new life in retirement. Many have lived in crowded, polluted city neighborhoods where recreational facilities were hard to find. Now they are searching for things they never had.

Housing experts say the earliest stage of retirement now starts with pre-retirees or empty-nester couples -- those 45 to 60 years old with grown children -- who usually seek a maintenance-free, "wide world of sports" life style.

This rapidly growing segment of the market usually is shopping for a smaller detached home in a tennis-or-golf-oriented country-club community.

Between 65 and 70, senior citizens gradually slide into the second stage of retirement. It usually starts when normal aging begins to accelerate and life-styles built around strenuous recreation may have to be forgotten.

"They aren't ready to sit around and vegetate, but their needs have changed," noted Lewis Goodkin, president of Goodkin Research Corp. "Everyday sessions on the links and courts become once-a-week, shorter sessions."

And keeping up the house becomes a chore. Entertainment is more on a congregate basis than individually planned, so the larger house is not needed. So comes the next move into something smaller, more compact -- probably a condominium community with more sedate activities, Goodkin said.

Between 70 and 80, still another life style takes place.

"Normal aging is taking its toll and it is becoming extremely difficult to cope with everyday living," Goodkin said. "The logical thing for them is to choose congregate housing for a life-care community. This will solve many of their problems, and in the process, it is yet another move into another housing unit."

At this stage, health of one or both members of the retired couple is the most important factor influencing housing selection, a nationwide survey on retirement life has revealed.

If a health change for the worse occurs several years after retirement, it often forces a couple, who are satisfied with their housing choice, to make a second big move. It could be back to an old neighborhood but more often it is a move nearer to a relative who can give some physical assistance, or closer to health-care facilities, the survey said.

Despite the benefits of some retirement communities -- balmy weather, beautiful landscaping, clubhouses, shuffleboard lanes, first-floor rooms and nearby top-quality medical facilities -- in the end those well up in their years often are left only with realities of life and death. CAPTION: Picture, AT HERITAGE HARBOR -- U.S. Home is the developer of a new adult community in the Annapolis area. This one-story dwelling, with two or three bedrooms, is one of the models being shown at Heritage Harbor, located on South Haven Road. The site is on a peninsula between Broad Creek and the South river. A community lodge is under construction. Prices of dwellings range from about $73,000 to $104,000. Initial occupancy began this month. By Jack Hayes for The Washington Post