If you think aerobics classes are only for young upwardly mobile professionals, a visit to the newly opened Washington Seniors' Wellness Center in Southeast will change your mind.

Long before class begins in the center's mirrored exercise room, about 20 seniors -- men and women ranging in age from 55 to 91 -- warm up to the sounds of Wham, the Jacksons and Hall and Oates. They are dressed in brightly colored leotards and skirts or sweat pants.

They giggle when Theressa Green, the petite, lively instructor, yells "Come on, give me some hips!" But their enthusiasm for the trunk twists, shoulder rolls and cancan kicks rivals that of any dance-exercise studio.

"They're so energetic," Green said recently after an early morning session. "They kill all the myths about old age."

Viola Malone, 55, who had just completed the workout, said, "It gets my morning off to a good start. We have fun in our sessions."

The city-operated Wellness Center, at 3857A Pennsylvania Ave. SE, is designed to help healthy seniors stay that way by promoting life style habits that improve and maintain health.

In addition to working out in the exercise classes, seniors attend nutrition lectures that address the dietary needs of the elderly. In a recent class on food additives, for example, seniors were taught how to read food labels and check for salt content.

The nutrition and exercise classes, each 40 minutes long, are offered six times daily on Monday through Friday, from 9 a.m. to 5 p.m. Seniors are encouraged to attend both nutrition and exercise classes twice a week.

The center also plans to offer programs on quitting the smoking habit, living without one's mate and managing stress.

There are 102,926 District residents 60 years or older, about 16 percent of the city's population, according to the 1980 census. Many of these seniors are recent retirees in good health, but "if they are not active and are not eating properly, we see a quick decline," said Linden Griffith, director of the center.

"We worked most our lives," said class member Alberta Jones, who looks much younger than her 65 years.

A group of women who had come to the center for the first time nodded in agreement. "We don't want to stagnate," Jones said.

E. Veronica Pace, executive director of the D.C. Office on Aging, created the Wellness Center to help seniors maintain their good health.

Pace said that when she took office two years ago, she began looking around at existing senior services. "What I was seeing is that our people were aging in place, that our senior centers were becoming places that were programmed around the moderately well and not-so-well elderly," Pace said.

"Looking at this, I began to wonder how we could make a difference from the preventative side," she said.

The Wellness Center, which opened May 2 as part of the District's Older Americans Month celebration, is funded by the Office on Aging and operates in conjunction with the East of the River Health Association.

Because it is a fitness program for seniors, the center closely monitors the physical condition of its participants. Those who sign up for the exercise class must fill out a medical questionnaire, talk with a nurse at the center and get a release from their doctor. An exercise program is tailored to each senior's individual needs.

Participants can monitor their blood pressure levels with a machine that stands in the lobby. The center's spacious building in the Fairfax Village Shopping Center also is equipped with a small medical station, and the two full-time and additional part-time staff members are trained in cardiopulmonary resuscitation (CPR).

The classes are free to District residents 60 years and older; nonresidents are charged a modest fee.

"I used to go to another center for exercise and diet and pay money I didn't have. I had to drop out," said Malone, who signed up at the center immediately after learning about it from a friend.

One goal of the Wellness Center is to help seniors incur fewer medical costs over the long run as the result of improved health. But the center also is a place where seniors can socialize, thus diminishing the isolation that often is a serious problem for many elderly people, Pace said.

As Alberta Jones noted: "It keeps you going, keeps you alive."