The fastest-growing segment of the population in Fairfax County is residents over the age of 75, a group that will place increasing demands on county services during the next several years, according to a new county report.

The study, conducted by the Fairfax County Area Agency on Aging and financed with an $18,000 federal grant, examined the status and needs of Fairfax residents age 60 and over.

There are now 74,000 residents who are 60 or older in Fairfax, and the number will rise to around 100,000 by the end of the decade, the report said. The agency attributes the group's rapid growth as a percentage of the population to the fact that people are living longer and that many who settled in Fairfax just after World War II are now elderly, officials said yesterday. There are now about 650,000 people in Fairfax County.

Fairfax residents between the ages of 60 and 74 tend to be married, well educated, have sufficient income and place almost no demand on county services, the report said.

The report revealed that those over age 75 were less healthy and far more likely to be poor, less-educated, single and female.

The group over age 75 increasingly will require services from the county for such things as adult day care, home maintenance assistance and transportation, the report concluded.

There remains a need for more county-run permanent-care facilities for the elderly, officials agreed.

The two facilities the county now operates "don't have anywhere near enough slots to fill the demand," said Henry Shenker, until two weeks ago the chairman of the Fairfax County Commission on Aging.

For the 33 percent of Fairfax's elderly who do not own their homes, Shenker noted, housing assistance is often the major need. "It is a major problem . . . . The elderly are less flexible financially, and they are getting it in the neck," he said.

The report does not estimate how much county money for expanded services will be needed to serve the growing elderly population, and Carla Pittman, director of the agency, declined to give a figure yesterday.

However, Anne Long Morris, director of the study, emphasized in an interview yesterday that the study surveyed noninstitutionalized elderly and that many of the report's findings show that small-scale, informal assistance would make it unnecessary to institutionalize many elderly citizens.

Such assistance includes in-home services to those over age 75, some of which could be provided on a volunteer or part-time basis by recently retired, younger residents, Morris said.