A group of elderly citizens in the metropolitan area have discovered that Ponce de Leon was not so crazy in searching for a "fountain of youth." They have discovered one in their own neighborhood pools, where almost every day they bob and job in water exercises, take swimming lessons or swim as much as 1,500 yards.

Among them are men like Eric Cattling, who swims half a mile three times a week in the Silver Spring Family YMCA pool. Catling, 77, who had a triple by-pass heart operation seven years ago, walks a mile in 30 minutes on the days he does not swim.

There are women like Nellie Brown of Alexandria, Va., a dauntless polio victim who has competed in the national Golden Olympics and in synchronized swim and masters swim meets. Not even a hip replacement last fall has kept her out of the water. Nellie is 87.

Sixty-six year old Sister Mary Martial is an example of older swimmers who work. Because of her job, she frequently must delay her swim until evening, but afterwards feels so energetic, she said, that she often does another three hours of work at home.

And once in a while, a senior swimmer comes along like Mimi Lee, wife of former acting Gov. Blair Lee of Maryland. At 60, she teaches swimming at the Silver Spring Y, volunteers in a swim program for Montgomery County Handicapped children, joins in D.C. Master Swim Team events, and is one of the county's ardent advocates of public swimming facilities.

The 300 senior members of the Silver Spring Y are typical of other older members of the Y. On weekdays they crowd the pool during open and adult sessions. At least 25 take water exercises and swim lessons, while others enroll in swimming and exercise classes or in advanced swimming and practices.

In the 1960s, physical fitness became a national passion among older Americans as well as the young. Instead of ambling, they strode along. They did calisthenics, bicycled and jogged, but few swam.

In 1972, the President's Council on Physical Fitness and Sports reported that 39 percent of the 60-and-over population exercised regularly, but only 4 percent of them were swimmers. By 1975, the U.S. Public Health Service found that half the population over 64 exercised regularly, with the number of swimmers rising to 11.8 percent. Today, pool managers agree that senior swimming is skyrocketing.

In Maryland, for example, nine months after the Prince George's County Department of Aging began an exercise and swimming class with 10 members, the number had jumped to 350.

Another indicator is that, while Montgomery County for years has had only one indoor pool, a second will open in August and funds have been approved for planning a third. The county's experience with its new outdoor pool in Bethesda undoubtedly influenced its decision to expand. By the end of last summer, there were more than twice the expected number of registrations. Of the total registrants, 1,466 were older swimmers.

Richard Manse, the physical education professor at Prince George's Community College responsible for the county's senior swim program, says it is popular because "senior citizens are smart. They would in no way have become involved, unless they saw its benefits."

The older swimmers say simply that swimming makes them feel good -- both physically and psychologically. They say their aches and pains diminish, they are more agile, breathing is easier and they have more energy.

D.C. orthopedist Peter A. Moskovitz explains that swimming, like walking, jogging and cross-country skiing, is an aerobic exercise in which a person's heart and lungs provide sufficient oxygen for muscle requirements on a "steady state basis." These activities, he said, are best for maintaining body tone and lung efficiency.

"The elimination of gravity (the body feels 90 percent lighter in water) is wonderful for people with lower back and knee problems that hinder them from bearing their body weight," said Moskovitz, "and the gentle, steady exercise of swimming is therapeutic for people with arthritis and bursitis."

C. Carson Conrad, executive director of the president's council, particularly favors water exercises, but emphasizes that continuing "practice is basic because of decreasing strength" among older people. The practice must be hard enough to raise the pulse gradually to 120 beats per minute, and should then gradually ease off before the swimmer leaves the water, he said.

Three residents of the Hebrew Home of Greater Washington in Rockville can measure their accomplishment with a clock. When they first went swimming at the near-by Jewish Community Center, they were wary of the whole idea; the walk there took 15 minutes.

Once accustomed to their weekly outings, however, the change was "fantastic," said recreation therapist Lois Cohen. Their self-image soared and they duplicated their water exercises in regular exercise classes. They are now so exhilarated by their swim, she said, that their return walk to the home takes just five minutes.

Pools in Montgomery County where seniors can swim and often enroll in classes include one indoor and three outdoor pools operated by the Department of Recreation; the Montgomery College pool; YMCA facilities in Bethesda, Gaithersburg and Silver Spring; The Rockville Municipal Swimming Center, and the Jewish Community Center of Greater Washington, located in Rockville. In addition, the Sheraton Inn in Silver Spring offers a discount to seniors who wish to use the motel pool.

In Prince George's County, the Park and Planning Commission operates two indoor and three outdoor pools, with three other outdoor pools under construction. Bowie State College operates a pool on Andrews Air Force Base.