It's 6:30 a.m., a good time for sleep. But Mort Glickenhaus, 65, is huffing and puffing on a treadmill in the basement of his Boston apartment building.

"My cardiologist says I never have a problem passing the stress test," he says. "Daily exercise keeps me fit and relieves tension."

By all accounts, Glickenhaus is doing the right thing. By exercising, the experts say, he is tapping into a natural fountain of youth that can stave off age-related problems like arthritis, osteoporosis and heart disease.

A recent study published in the American Journal of Public Health confirmed the benefits of an active lifestyle for seniors. A six-year look at nearly 5,000 men and women over age 65 showed that vigorous physical exercise cut their risk of death by one-half to two-thirds after three years and reduced the likelihood of their developing disabling physical problems.

Today there are more than 50 million Americans over the age of 55 and increasing numbers are participating in activities that range from exercise walking to weight training.

They're swimming and jogging, playing tennis and golf, and taking aerobics and yoga classes. They are finding that they have more flexibility and strength, more energy and even sleep better at night.

Dr. Maria Fiatrone has reported that 100 residents of the Hebrew Rehabilitation center for Aged in Boston who participated in a four-year study more than doubled their leg-muscle strength using exercise machines. If 85-year-old residents of a retirement home can successfully take up resistance training, you know it's never too late to begin and benefit from an exercise program.

Whether you choose weight training or walking — or both — when you first start out, start slow and be consistent. Dr. S. Terry Canale, a sports medicine orthopedist and chief of staff at Campbell Clinic in Memphis, Tenn., emphasizes that it's important to do something physical every day.

He offers advice for beginning exercises that suits any activity:

*Don't start an exercise program that overwhelms you.

*Choose activities you like. Don't force yourself to do something you don't want to do.

*Combine exercise and social time. Walk or go with a friend, join an exercise class, participate on a team.

*Make exercise a normal part of your daily routine.

*No pain, no gain is out. Listen to your body. If you feel pain or fatigue, lighten up.

*Warm up and cool down gradually with stretches and slow movements.

*Use proper equipment, especially shoes.

Seniors should also consider combining activities for the best overall effect. Even if you are involved in some kind of aerobic activity, you may not be fully strengthening your body. Fitness experts are recommending that weight training be incorporated into an exercise program to avoid the weakening of muscles that comes with age.

But before determining your activity of choice, check with your doctor. Aerobic exercise alone may be enough of a workout for people with mild hypertension. Strength training, which counters muscle loss and builds bone density, may be a good addition for others.

One of the easiest and least expensive forms of exercise is walking. And not only does it get your heart pumping, it can be a time to catch up socially with friends.

Several times a week during non-winter months, Evie Glickenhaus, Mort's 60-year-old wife, has a standing date with a friend that takes them from a busy Boston street to a lush reservoir path that they circle twice before heading back. Door to door, they walk six miles.

"We talk and the time goes by very briskly," she says. "It's my social time of the day and my exercise time."

University of Florida researchers found that men and women ages 60 to 79 who participated in either a moderate- or high-intensity 45 minutes walking program for six months significantly reduced their blood pressure.

Another study conducted by researchers in North Carolina monitored the sleep of a group of older adults. Those who were involved in aerobic walking or other aerobic sports such as jogging, tennis, swimming and bicycling, fell asleep sooner, woke less during the night and slept more deeply than their sedentary counterparts.

While walking is a pretty safe activity, fitness experts nonetheless recommend a good five-minute stretch before and after a walk that focuses on the Achilles heel, quadriceps, hamstring and groin.

Of course, stretching doesn't have to be simply a prelude and conclusion to a workout. Hatha yoga, which incorporates gentle movement with breathing and relaxation techniques, can help build strength, flexibility and stamina.

If you can't get to a class or prefer to work out in the privacy of your home, check out what's available in the local video or bookstore.

Another way to get the heart beating faster — and increase flexibility — is to do low-impact aerobics. Most communities across the country have a variety of class offerings through health clubs and senior centers.

Whether you go for a morning swim or evening jog, or just a walk around the block, the bottom line is to choose an activity or combination of activities that you enjoy and will participate in regularly.