The door to the Volunteer Services Unit in the Arlington County Courthouse swung open and Dorothy Lake sauntered in, shaking rain from her flowered umbrella.

"One more day like this and I'm going to be covered with mildew," she announced with a rich, throaty laugh before sitting down to a stack of blank envelopes and an electric typewriter.

Addressing envelopes is the sort of clerical work Lake does twice a week for the county, the sort of volunteer work she has done continuously since retiring at age 70 from what was then the U.S. Department of Health, Education and Welfare.

That was 18 years ago.

Today Lake, at 88, demonstrates a demographic statistic and a personal style that are forcing Arlington, as well as other localities, to redefine the people called "elderly" and the services they need.

From 1970 to 1980, when Arlington's population as a whole dropped by 12 percent, the number of county residents over 65 jumped by 30 percent. Arlington's oldest residents, not the young professionals who have received a lot of public attention, showed the largest proportional increase in numbers.

Human services officials say that this growth, and an awareness that the "elderly" population may include persons from 65 to 105 with widely varying demands, have created a need for new and expanded services.

The changes in Arlington's population mirror demographics nationwide. Between 1970 and 1980, the entire U.S. population grew 11.4 percent, while the number of people over the age of 65 rose 27.3 percent. The fastest-growing age group, those over 85, jumped 48.3 percent.

"We have every reason to believe that by the year 2000 we're going to have even more elderly people than we have now," said William H. Van Hoose, professor of education at the University of Virginia. "No other age group is growing that rapidly."

Van Hoose, director of a new aging studies certificate program that trains students to work with the elderly, said those population figures have created new demand for services and employes.

"The psychological-social service type help is where there's going to be the need," he said. "The need is going to be in nursing homes, senior citizen centers, recreation programs and housing."

In the lingo of social service professionals, what is needed is a "care continuum." In practical terms, that means services flexible enough to give older people exactly what they need as their needs change: housekeeping service, a daytime companion, or just someone to help with a sticky zipper now and then.

"I think the single most important fact that we have to keep in mind is that you can no longer talk about the elderly population in general," said Arlington County Board Chairman John G. Milliken.

"A senior who is 63 years old is not comfortable being lumped in a group with a senior who is 78," agreed Elinor Ginzler, volunteer program assistant in the county Citizens Assistance and Information office.

"You want support, but you want it not to be offensive," said Arlington's director of human services, Martin P. Wasserman. "You have to have a system that recognizes the aging process. It's very expensive to be that sensitive."

The design of services for the diverse and growing population of people over age 65 will be a priority for Arlington, Milliken said.

"If you were sitting down with a clean sheet of paper and said, over the course of the next 10 years, what are the areas for redefining needs, [the elderly] would be, if not first on my list, then certainly right up there," he said.

In recent years, services for the elderly have swelled in number and popularity. Meals on Wheels delivers more than 150 hot lunches per day, brought by drivers who are often senior citizen volunteers.

The Madison Center, where a "day care" program of activities for frail elderly persons began in 1976 with 11 participants, now serves as many as 40 a day. Three federally subsidized high-rise apartments for elderly tenants, with a total of 745 units, have two- to three-year waiting lists.

As the county's population and housing stock age, the lack of affordable and appropriate places to live will become the most crucial gap in services for Arlington's elderly, according to county officials.

"A lot of people came to the D.C. area in the 1940s to get government jobs," said Ruth Ullom, supervisor of the Madison Center. "Their families are far away. They never went home. They are old now, and they are loners."

"The projects that were built originally for populations that were hardy are now becoming projects housing a growing number of people who are frail," said County Board member Albert Eisenberg.

Eisenberg has proposed a county-sponsored "sheltered housing" program as one way to adjust to that change. Under his plan, several apartment buildings with large numbers of elderly residents would be assigned a staff coordinator to evaluate the services they currently receive and arrange for additional services such as personal care, housekeeping or transportation. Clients not eligible for free services would pay for them according to a sliding fee scale.

The County Board has endorsed the concept and asked the staff to develop a detailed plan by the end of the year for putting the idea to work.

"The whole purpose of this is to keep people at home in familiar surroundings . . . to avoid unnecessary, unhelpful institutionalization," Eisenberg said.

While institutionalization is viewed by many as an increasingly unpopular and expensive extreme for elderly persons, volunteer work for healthy, active retirees represents the opposite extreme, one that county officials say will be more in demand as people both live longer and retire earlier.

David Kwass, who will say only that he is "over 65," retired in 1978 after 30 years as a legal consultant for the Veterans Administration. Since then, he has volunteered as a legal counsel for the elderly, served as "companion" to a man in his mid-60s, assisted a Swanson Intermediate School teacher with special education students and led a Great Books class for gifted children.

Kwass now works two afternoons a week at the information desk in the Arlington Courthouse, fielding queries from residents on every subject from property taxes to dog licenses. Also, he guides visitors at the Museum of American Art, the Museum of American History and the Kennedy Center.

"I believe very firmly that you have to continue being active," Kwass said of the routine that keeps him busy four days a week and often occupies his evenings with "homework" on new museum exhibits. "It's very important not to sit and deteriorate."

Volunteer coordinator Jean Berg said such feelings are the norm more often than the exception.

"After [recent retirees] have played golf until they're ready to fall down and gone on the trip they've always wanted, then what do they do?" she said.

Many retirees who contact the office have spent decades working in giant federal agencies and are eager for work of a different tempo, said Ginzler.

"You can do something outrageous; you can be a hot line listener when you were a budget analyst . . . . Some, it's very clear, just need to be needed," Ginzler said.

Demographics have created increasing numbers of elderly generations within one family, Ullom and others said. Eight of the Madison Center's participants are in their mid-nineties, with sons and daughters in their seventies.

"There's a lot of resentment" from elderly persons with elderly parents, said Ullom. "Some people feel: 'Hey, I thought when I was this age I wouldn't have anyone to take care of, and here I am still stuck with this parent.' "

On a recent morning, two staff members at the center led a group of adults, many of them wheelchair-bound, through a musical exercise routine of rotating their wrists, waving their hands and bending their knees.

"We alter the event or the activity so they can participate," said Ullom. "Our poker game is much slower; sometimes we bend the rules."

The biggest problem at the Madison Center, a concern echoed by many social workers, is the task of reaching elderly people who may need help but do not know where to find it.

"Our track record with people living alone, which is a group we feel we really need to serve, is not very good," said Ullom.

Lois Atkins of the Bureau of Services to the Elderly and Handicapped said her office received 47 calls in the first three months of this year from friends or neighbors of elderly persons reporting alleged abuse, neglect or exploitation. After investigation by staff, 26 of those reports were determined to be founded, she said.

"By the numbers, we know there are enormous numbers of people out there who aren't making use of the services, who don't know who to call," said Terry Lynch, supervisor of the Agency on Aging in the Department of Human Services.

Many elderly adults may need help but "think that kind of [social service] program is for somebody else," said Atkins. "We just hope we hear about these people."