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In 1990, one in 10 Americans was over age 50. By 2000, the proportion will rise to one in five.

The age boom is coming — and not without side effects. Osteoporosis, hypertension, diabetes and other maladies linked to aging are headed for a steep incline in the next half-century.

If we ever needed an anti-aging wonder drug, we do now. Fortunately, it looks like we may have a "drug" that doesn't even require a prescription: regular physical activity.

Unfortunately, even though most of us have ample access to it, few of us seem to be using it.

Regular physical activity has been shown to fight the destructive effects of getting older.

Study after study asuggests that older adults have more reason to exercise than any other age group. But for some reason, mature adults just don't exercise as much as younger adults.

According to the 1996 U.S. Surgeon General's Report on Physical Activity and Health, the percentage of Americans between 45 and 64 who participated in regular sustained activity actually decreased from approximately 22.4 percent to 19.15 percent between 1991 and 1992.

Yet the arguments for exercising are persuasive.

Exercise is one of the little-known foes of osteoporosis. This weakening of the bones is common among postmenopausal women. Treatments for osteoporosis are still being developed, and range from somewhat experimental estrogen replacement therapy to simply increasing calcium and vitamin D intake.

According to the Surgeon General's report, the more pressure applied to bones, the stronger they become.

Regular weight-bearing activities such as walking, lifting weights and jogging are good ways to fight the effects of osteoporosis before it gains a foothold.

If you have a family history of osteoporosis, or are nearing menopause, you should consult a physician and look into starting a simple exercise program. While regular physical activity may not be the cure-all, it has been shown to increase bone density.

High blood pressure affects an estimated 50 million to 60 million Americans and can cause serious problems, including cardiac arrest, arterial clotting, stroke, aneurysms and other vascular disorders.

Several prescription drugs are available for treating hypertension, and a consultation with a physician is worthwhile if you think you're at risk. Fortunately, hypertension can also be treated easily with exercise.

According to Ralph LaForge, a spokesman for IDEA, the International Association of Fitness Professionals, hypertension is easier to manage through physical activity than most other age-related disorders.

"That doesn't mean it's easy to manage," says LaForge. "But the response in most studies shows that after burning just about 600 net calories a week more, most people tend to start quite significantly dropping their systolic and diastolic blood pressure."

The bottom line: Regular exercise, teamed with a doctor's recommendations, can hold at bay the risk for a crippling case of hypertension.