When Lou Azzarano and Lam Tieu meet in the cafeteria line at Fairfax County's Groveton Elementary School, their hands go out and they "slap five," street dude fashion.

It's a common greeting at ethnically rich Groveton in the grittily urban Rte. 1 corridor south of Alexandria. But there's one unusual thing about this lunch-time encounter: Lam is 12; Azzarano is 70.

But are at Groveton -- Vietnamese-born Lam, who fled his country with the Boat People, as a fifth grade student in the county's English-as-a-Second-Language program; Azzarano as an ex-Marine who speaks a handful of languages and is staying busy in retirement.

"It helps me keep young," says the gray-haired, volunteer tutor. "If I weren't doing this, I'd be at home listening to my arteries harden."

For years, Fairfax seemed forever young, a fast-growing bedroom enclave for thousands of young Washington suburban professionals and their families. School buses, station wagons and tricycles were its everyday symbols.

But after three decades of greening, Fairfax is starting to show some gray. Census figures show the county's elderly now account for a sharply rising proportion of its 600,000 residents. At the same time the school-age and preschool population is dwindling.

In a major demographic shift, the number of children in Fairfax under age 5 decreased 12 percent between 1970 and 1980, according to census data. During the same period the number of residents over 55 shot up more than 100 percent.

This trend, which officials say is also surfacing in other Washington-area jurisdictions, appears to signal significant, perhaps dramatic, challenges ahead in a county that last year, despite some school closings, spent nearly half its general fund on education.

"The school system will have to answer to the elderly," says Fairfax Superior Marie B. Travesky. "It will have to make accommodations."

In a limited way, it already has.

At Groveton, in classrooms where children once sat, elderly residents like Azzarano play bingo and cards, listen to talks on health care and nutrion, knit and crochet, and to the delight of principal William Zepka and his staff, help the children with their reading.

One regular, Villeret Camp, who is 69 and lost a leg to cancer, is picked up daily by a county-subsidized van and taken to Groveton. In the morning she socializes, plays games and listens to the discussions, but after the lunch break she becomes a volunteer reading assistant.

Sirron Foster, 8, who cuddles up to Mrs. Camp as he reads aloud from his primer, says, "She helps me with words I can't pronounce."

Fairfax School Superintendent Linton L. Deck, whose constituents are approximately 200,000 parents, agrees that "we may need to rethink the basic mission of the school system" as enrollment falls. "Should we look at ways to serve a broader constituency? The answer, I think, is probably yes." w

But the scope of problems in housing, real estate tax burdens, zoning, health and transportation that affect the county's growing elderly population is only beginning to be perceived, some say. One privately run nursing home in the county says it has a one-year waiting list for beds paid for by Medicaid.

Superior Thomas M. Davis III -- the youngest board member at 31 -- believes he has been the future already in the Mason District he represents.

At a recent constituent meeting, the elderly turned out in force to complain about soaring property taxes and the size of an education budget that continues to grow even as the student population declines and schools close.

"There were about 175 people," Davis says, "and I asked all those who had kids in public schools to raise their hands went up.

"I asked how many had kids in private schools. Ten hands went up. That means that about 160 didn't raise their hands. We just don't have kids anymore in Mason District."

Supervisor Travesky doubts, however, that most older people want to leave the county. "Many have decided not to go to Florida and vegetate," she says. "This is their home and they want to stay here."

Both board members are aware of one activity among older residents: voting."I'm seeing the clout of the elderly," acknowldeges Travesky.

There is pressure for better public transportation in older neighborhoods, most of them inside the beltway, say board members, while younger suburbanites lobby for express transit to and from their own subdivisions, mostly in the outer suburbs.

But no issue rivals housing in importance in the minds of many older constituents, whose fixed incomes make them vulnerable to Fairfax's expensive real estate market and shrunken supply of affordable apartments.

"If this [high property taxes] keeps up," says Virgil M. Rogers, 82, a retired school administrator and widower, "the supervisors are going to force a lot of us oldtimers to join forces to oust them."

Two Fairfax board members, Marytha V. Pennino and James M. Scott, say they think it's time to consider easing restrictive zoning laws that out-law so-called accesory apartments -- an arrangement that would permit elderly renters to band together in large houses to share fuel and utility expenses.

The apartments, common in some older cities in the Northeast, are popular because they private housing for the elderly who want a smaller place to live but don't want be alone.

But housing officials say efforts to modify the county's zoning regulations or build federally subsidized housing are likely to run up against another interest group -- the child-rearing, middle-aged couples who seem ever-alert to any changes that threaten their suburban serenity.

When one church group, the Arlington Assembly of God, announced plans to construct a subsidized housing project for the elderly in the Burke section of Fairfax, many of the young residents of nearby subdivisions attacked the move. One resident, Burton Rubin , said he feared the project would become a rehabilitation center for drug addicts and alcoholics.

The project was approved only after Supervisor Travesky intervened and soothed homeowners' fears.

Nursing homes, which give shelter to some, are an expensive alternative and out of reach for most for average care at Fairfax Nursing Center, typical of many such homes in the county, is about $1,350.

"You either have to have a ton of money or nothing to stay in a nursing home," says Ann C. Hall, administrator of a home near Vienna.

Those who lack the funds can apply for Medicaid assistance, but many nursing homes take only a few such patients because the federal-state program does not reimburse for some services.

"We have a one-year waiting list for Medicaid beds," says Hall, adding that she gets a minimum of three such calls a day.

Dr. Otto A. Kurz, who has many elderly patients in his practice in Falls Church, says a state and federal crackdown on boarding houses has forced many of them to close, drying up an important source of housing for the elderly.

"A good number of nursing home residents don't belong there," he says. "They could live elsewhere, in the a home for adults, where the cost would be about $500 a month."

For his part, Supervisor Tom Davis says he began to grasp the plight of the elderly when his mother decided to move to a cheaper home in Delaware. A retired secretary who had lived for 25 years in eastern Fairfax, she was forced to sell her house because she could no longer afford the increasing property taxes on her home.

"It was just too tight for her," Davis says. "She had to go."