It's never too late to start an exercise program, says Dr. Andrew Goldberg of Johns Hopkins University.

"We just started a program for 76-year-olds, and we're planning one for 80-year-olds," he says. "We feel the elderly should exercise. They can start at 70 or 80, but they should be screened and initiate exercise in a controlled setting."

Experts offer the following advice for those who want to be fit after 50:

*Most people over 50 do not need a $200-$300 exercise stress test. Dr. Sam Fox, professor of medicine and director of the preventive cardiovascular program at Georgetown University, says an inexpensive risk factor analysis can usually be used instead. Such an analysis, available from the American Heart Association (337-6400), offers a fairly accurate prediction of who is likely to get heart problems. People at risk -- such as those with a family history of coronary artery disease, or those who are obese, smoke cigarettes, have high blood pressure or high cholesterol -- may benefit from a stress test.

*Intensity of exercise, while crucial to good heart health, may be a cause for concern. "If a person is obese and a heavy cigarette smoker, it's bad news for exercise," notes Fox. "The older the person, the more sedentary and the higher the level of anticipated activity, the more we would tilt toward getting a stress test clearance.

"If they say, 'I would like to go out and start my kids playing tennis,' I say go ahead but take it easy if you notice any discomfort. But if they say, I used to play squash, but I haven't played for 20 years -- and there's no way you can play squash without intensity -- just on the basis of the intensity, I'd say you might want to take a stress test."

*Be especially attentive to old injuries that may not have healed properly. Rehabilitate the old injury first with strength exercises, doctors advise.

*Your target exercise heart rate can be calculated by the following formula: Subtract your age from 220. This is you maximum exercise heart rate. The target range is 60 to 85 percent of that maximum, depending on your condition. So a 55-year-old would have a maximum of 165 beats per minute and a target range of 107 to 140.

*Be aware that women in general, and especially older women who haven't exercised for a number of years, are frequently prone to knee injuries, bursitis or tendonitis. Strength and flexibility training may help.

*Be careful about using exercise videos, which may be aimed at a very fit general audience and may not be based on sound exercise principles.

*Make sure your instructor is well-trained in physiology and anatomy and points out the proper way to do the exercises.

*Consult your physician before starting any new exercise program.