There are two main ingredients for aging well, according to Washington gerontologist Dr. Jack Kleh: functional health, and a reason to get up in the morning.

Now many, regardless of age, find it difficult to meet both these requirements simultaneously. But Washington is full of people who do.

With an increasingly older population - the government estimates by the year 2020 the number of Americans 65 and over will be double those ages 40-49 - more and more people will have to find a satisfying "reason for getting up in the morning."

"I don't want to rust out. I want to wear out," says Uriah O'Neal, 74. He drives a cab four days a week.

O'Neal came to Washington in 1945 from North Carolina. Since then he has worked as a laborer, cook, and for most of those intervening years as a power-plant controller for the federal government.

O'Neal says there are two reasons he continues to work: "I like what I do, I couldn't sit around all day. And my pension is not enough to take care of us."

O'Neal lives in his own Capitol Hill home with his wife, a former beautician. Nearby are his four daughters - one of whom is a doctor - grandchildren and great-grandchildren.

Even in these days of the much heralded collapse of the family, contact with relatives and a sense of community emerge as vital to one's well-being, particularly in later years.

"Very few people ever move when they retire, only 5 percent ever leave their homes," says Mary Powers of the American Association of Retired Persons.

"It's important to have contact with people you know. But that's harder to develop when you get older," says Kleh, who tells his patients to think twice about pulling up stakes and racing off to Arizona or Florida.

A few of his patients have come back happily to Washington after dissatisfying retirement moves.

One point many older Washingtonians made in favor of staying is that even if they aren't natives, their social ties are here.

"After being away for so long all the connections get broken," says Patrick Hayes, 70, a founder of the Washington Performing Arts Society. "There's no one in Boston for me now, they're all here. When I go to the Press Club for a drink, I know all the bartenders. I feel at home."

Hayes considers himself blessed that his vocation and avocation are the same. His work requires that he attend, say, concerts by Luciano Pavarotti.

"It's a lovely town to be old in. Mind you, I'm not saying I'm old. There are 20-30 small galleries you can walk into, free. The Phillips Collection has concerts on Sunday, also free. And you could spend 70 hours a week at the Smithsonian; it's a massive university."

Hayes has been jogging religiously since he was 50, an exercise that allows him to indulge in pecan rolls at lunch.

He also credits his good health to a positive attitude about the future. One Saturday afternoon, while he was the only one at work in his downtown office, he gazed across Washington and described what it would look like years from now when all its cultural apparatus is in place. He added that he plans to be around.

Carolyn Agger, 70, a partner in the Arnold and Porter law firm, credits her fortitude to her forbears:

"Some people are fortunate enough to inherit a good supply of energy. My family lived to be old and active.... But it's getting to be appalling the number of people in my college class who've died."

The cigar-smoking tax lawyer ("I used to smoke cigarettes heavily") has cut down on the time she spends at the office. She now works about 50 hours a week, down from a six or seven-day-a-week routine of working until midnight. She said the passing along of her skills allows her to lighten her load:

"I like to do the nitty-gritty, go to the books. But as I get older, the younger people can do it just as well, if not better. I don't do all the hard drafting; that's what they're here for. And it's a thrill to see a new lawyer learn to write a brief."

Agger has a large circle of friends she sees frequently, but has no regrets that she and her husband, former Supreme Court Justice Abe Fortas, have no children.

"I'm glad I don't when I see the awful time my friends have had with their children. They're beginning to straighten out now, but it takes years...no, I have dogs."

While there are those who do retire happily and take up an avocation full time, be it gardening or photography, there are others who find a hobby should remain just that - a hobby.

Charles Schafer, 71, who retired from the FBI in 1971 after 31 years with the bureau, "loafed" in 1972.

"I played golf, went on cruises," he recalls. But he found a steady diet of loafing gave him the "willies."

So after a stint as an investigator for the Department of Housing and Urban Development, he joined the American Association of Retired People as a full-time paid consultant for their crime-prevention program.

Schafer said many of his FBI colleagues have also found ways to use their skills in second careers. Several teach criminology at local colleges, or do consulting work for police departments.

Kleh finds his patients who have a continuing involvement with what they did professionally are the most satisfied. He also feels Washington is a good town for "recycling" careers or putting administrative skills into charitable and cultural organizations.

Dr. Edward Mazique, another physician whose practice consists mostly of older people, said he is an "exponent of changing your life style, not retiring." At 65 the robust Mazique is pondering his own advice.

"I have cut down the size of my practice to permit me to have my evenings free. But I love what I do. The day of the February snow storm, my wife said if someone didn't come get me out of the house she'd divorce me. So one of my patients with a four-wheel drive came and took me to the office."

Mazique, a native of Mississippi, said winning civil-rights battles earlier in his career - including the integration of Washington hospitals - has enabled him to enjoy a sort of "second life" now.

"I was not treated as a first-class citizen for many years. Now when I travel around the country, and back to Mississippi as a first-class citizen, I see things with new eyes." CAPTION: Picture 1, Dr. Edward Mazique; Picture 2, Carolyn Agger; Picture 3, Patrick Hayes