IF YOU COULD LIVE TO 150, how would you live your life? Would you start a new career at 65? Put off having children until 75? Be more concerned about long-term environmental changes?

Those questions are not something out of science fiction. In the last few decades, a seismic shift in health and longevity has occurred. Consider these facts: The fastest growing segment of the population is the 85-plus category. Between 1980 and 1990, the number of centenarians doubled.

"Age is inevitable, aging is not" is the credo of a group of doctors practicing the new specialty of anti-aging medicine. Dr. Ronald Klatz, below, is the president of the American Academy of Anti-Aging Medicine, whose membership has grown from 12 to 1500 in four years.

From 1960 to 1990, death rates from heart disease were more than halved, and stroke deaths plummeted by 65%. In this century, the average life span has risen from 47 years to 76, mostly because of improvements in sanitation and the elimination of infectious childhood diseases. But since the 1960s, people have been living longer because medical science has gotten better at treating chronic conditions like high blood pressure and heart disease. With continuing advances over the next 30 years, today's 76 million baby boomers can look forward to celebrating their 100th birthday in good health, and life spans of 120 to 130 years may not be unusual.

In 1993, a dozen physicians and researchers formed the American Academy of Anti-Aging Medicine, or A M, in Chicago. Forty persons attended our first scientific session. We now have 1500 members.

These doctors and scientists share one fundamental belief: Age is inevitable; aging is not. The aging process can be slowed, halted or even reversed. It's already happening in lab animals and in people. For example: Mice whose caloric intake has been severely restricted live twice as long on average and have far less cancer and other diseases. In humans, growth hormone has turned back many of the effects we associate with aging.

If you have had your cholesterol tested, taken a lipid-lowering drug, had a mammogram or taken replacement therapy for thyroid, testosterone or estrogen hormones, you have encountered anti-aging medicine. This approach is based on early detection, prevention and reversal of age-related disease. Ninety percent of all illness is due to the degenerative processes of aging. This includes heart disease, most cancers, adult-onset diabetes, stroke, high blood pressure, osteoporosis, osteoarthritis, autoimmune disease, glaucoma and Alzheimer's. With easy detection and appropriate intervention, most of these diseases can be prevented, cured or have their downward course reversed.

What is anti-aging medicine? Here are the basic strategies, already in use, for lengthening life:

Regular, vigorous exercise is the closest thing we have to an anti-aging pill right now. A study of 17,000 Harvard alumni found that men who burned 500 to 1000 calories a week (the equivalent of walking five to 10 miles) had a 22% lower risk of death, while those who expended 2500 calories a week added one to two years to their life span. Aerobic activity and weight-training raise levels of the body's master hormone that controls aging.

Diet. Nutrition is one of the greatest weapons against disease. The biggest health problem facing Americans today is obesity. Keeping daily fat intake below 30% and cholesterol below 220mg will cut your risk of heart disease. Eating five servings a day of fruit and vegetables lowers your chances of getting cancer. And a recent study found that eating nine to 10 daily servings of fruit and vegetables with three servings of low-fat dairy products was as effective as medication in lowering high blood pressure.

Antioxidants. Free radicals are highly reactive bits of molecules formed during the process of converting oxygen and food into energy. Like tiny grenades, they can damage cells and DNA. Many studies support the idea that free-radical damage contributes to age-related illnesses, like cancer and heart disease, and to the aging process itself. Antioxidants such as vitamins C, E and carotenes protect the cells by neutralizing the free radicals and may help prevent disease. For instance, a study at the UCLA School of Public Health found that men who took 300mg of vitamin C daily had a 45% lower risk of heart attack than those who took less than 49mg. And vitamin E lowered heart-attack risk by 41% in a large-scale Harvard study.

The best way to get your antioxidants is from a diet rich in fruits and vegetables, which also contain hundreds of cancer-fighting elements called phyto-chemicals. But most Americans don't have a diet rich in fruits and vegetables; they may need to take a daily vitamin-antioxidant complex.

Human growth hormone. Manufactured in the pituitary gland of the brain, this appears to be the master hormone of aging. Starting at age 20 or so, it begins to decline about l4% each decade. That decline is accompanied by many of the miseries associated with aging, from saggy skin to a potbelly to lack of vitality. But what would happen if growth hormone could be replaced?

We have reviewed many of the 28,000 studies that have been published on growth hormone worldwide in the last 10 years. These studies suggest a wide range of effects when our levels of growth hormone are elevated by a synthetic form (HGH): increased muscle mass, higher energy levels, enhanced sexual performance, regrowth of vital organs, restoration of youthful immune function, stronger bones, lower blood pressure, faster healing of wounds, smoother skin, regrowth of hair, sharper vision and elevated mood.

In a 1994-96 study of HGH, the largest of its kind, conducted by Drs. L. Cass Terry and Edmund Chein, an analysis of 202 patients who used it for more than six months found that 88% reported an increase in muscle strength, 72% a decrease in body fat, 71% an improvement in skin elasticity, 75% (of men) greater sexual potency, and 84% an increase in energy.

The FDA has just approved the use of HGH as a treatment for adults with deficiency as a result of pituitary disease, hypothalmic disease, surgery, radiation therapy or injury. Growth-hormone replacement for older people who are deficient should only be done under the supervision of a doctor experienced in its use. Formerly, it was allowed only to promote growth in deficient children.

The effects of HGH are still being studied, but anti-aging doctors anticipate that, not long from now, rejuvenating our growth-hormone levels may be as easy as taking vitamins. At least six major drug companies are racing each other to develop oral secretagogues - compounds that Stimulate the pituitary gland to release more growth hormone. In clinical trials published in scientific journals, a single 25mg dose taken once a day raised the hormone to youthful levels in people aged 64 to 81. More extensive trials will be needed before FDA approval, which may take three to five years.

But even now you can get many of the age-reversing benefits of HGH by stimulating your own hormonal levels naturally through diet, exercise and supplements. One study has shown that moderate to intensive aerobic exercise can increase growth hormone 1 1/2 to 2 1/2 times, while high-intensity weight-training can increase levels three- to four-fold. Finally, you can maximize the rejuvenating effect of exercise by losing weight if you are over your ideal body weight, following a diet that is low in fat and high in healthy proteins, and avoiding sugary, starchy foods.

In the future, your anti-aging doctor will develop personalized programs to ensure that you don't suffer the problems of aging. This year, the American Academy of Anti-Aging Medicine will begin certifying 400 physicians in this exciting new field. We believe it will become the dominant primary-care specialty in our society.

A personal trainer is not a luxury, it's a great investment in your health and well being! Ask any fitness expert and the answer is always the same. The best way to get started on an exercise program is to use a personal trainer.

BY DR. RONALD KLATZ AND CAROL KAHN