There is a quiet real estate investment boom under way across the country that shows signs of turning into one of the fastest-growing sources of new American housing in the coming decades.

Major corporations such as Marriott Corp. are plunging into it. Individual investors looking for tax shelters and long-term capital gains are sinking dollars into it. Community hospitals, health-care organizations, doctors and dentists are eager to be part of it.

It is the boom in specialized housing communities designed for the fastest-growing segment of the American population in the next 40 years: the 65-years-and-older set.

Forget about Yuppies, first-time buyers and other wet-behind-the-ears segments of the real estate market, said speakers at a national gathering of 700 investors and health-care industry executives here last week. The housing challenge -- and financial opportunity -- for the balance of this century is in retirement communities that include continuing medical care, food and other services at a reasonable price.

The demographics behind this message are powerful: More than 28 million Americans already are over 65, and the number will double early in the next century. The proportion of retirees over 85 years of age will quadruple during the same period.

* The majority of the rapidly growing "new elderly" don't want to move in with children. They don't want granny flats or mother-in-law suites. They want their own, independent living unit, ideally as close to their children and longtime family friends as possible.

* Although they are living longer, the majority of the new elderly are not free of health-care needs. To the contrary, the growing ranks of independent-minded 80- and 85-year-olds have significant concerns about their medical, skilled-nursing and food-service needs. The older they get, the progressively greater their requirements.

* No single age bracket of the American population has more financial clout than seniors 65 years and older. Many are well off, particularly those who sell their longtime homes and qualify for tax-free cash up to $125,000.

Roll these demographic facts together and you come up with an inescapable conclusion: In the words of Aaron Rose, head of retirement-center consulting for the accounting firm of Laventhol & Horwath, investment in housing construction that meets the needs of this population group "is about to go off the charts."

Rose estimates that, in the next five years alone, $33 billion will be raised and spent by private developers, limited partnerships and nonprofit organizations to create more than 1,800 multicare communities averaging 300 residents apiece.

Unlike the single-family retirement subdivisions that predominated in earlier years, the new boom will be in multifamily rental, condominium and cooperative communities that include restaurant, cultural and nursing-care facilities -- plus a relationship with a nearby hospital.

The coming wave of specialized, competitive housing communities will offer elderly residents a wide menu of "levels of care" to choose from, according to Rose and others.

Unlike high-cost projects that require residents to fork over large, nonrefundable cash entrance fees, many of the new generation of retirement communities will be purely rental, providing nursing- and extended-care services on a pay-as-you-go basis.

Typical of the trend is Island Lake Village, a 15-acre multilevel care community just completed near Orlando, Fla., by Oxford Development Corp., one of the country's largest rental-property developers.

The project has town house "cottages" and low-rise apartments that come with a choose-your-own-mix of services. Residents can take the standard package, including one meal a day prepared for them, all housekeeping and laundry, transportation, on-site medical assistance, and pool, game-room and cultural facilities.

Or they can go for a heavier package, including three meals daily, plus specialized bathing, dressing and other forms of personalized care.

Base rents at the project start at $1,000 a month for a single resident in a one-bedroom apartment and rise to $1,650 for a couple in a two-bedroom, two-bath unit.

Multicare projects such as this "are where an unbelievably large number of Americans are going to live long lives," says J. Stephen Eaton, head of an Atlanta-based firm that raises equity capital to finance them. In effect, Americans' longevity and desire for independence is stimulating the creation of a unique real estate product: housing that turns on services or turns them off as you need them and when you want them, he observes.