Gerontologist, those scientists who study aging, have been trying to unravel the mysteries of aging. Aside from normal wear and tear, they have identified three primary mechanisms, one genetic and two chemical, that lead to the body's breakdown. Genetically, the cells that compose us seem programmed to have a finite life span. All cells, whether from humans or worms, are able to reproduce themselves only a certain number of times. After that their metabolic functions begin to　deteriorate, their membranes weaken and they (and, eventually, we) die. Researchers still don't know what drives this cellular timetable. Until they can find a way to overcome it, the lives of humans seem to be limited to about 120 years.   Unfortunately, two types of chemical reactions conspire to reduce our actual life span well below that theoretical limit. The first is called free radical oxidation. Like an electric power plant, the body produces waste products as it burns its fuel (food) for energy. These wastes are called oxygen free radicals: highly reactive oxygen molecules that bond with virtually any biological substances they come into contact with. When free radicals bind to proteins and membranes, they weaken tissues and internal organs. When they bind to DNA, they can produce cancer-　causing mutations. 　The second destructive mechanism is called glycosylation, a process whereby sugars in the blood stream coat proteins, causing them to stick together and bind to places they normally wouldn't. This, in turn, stiffens joints, blocks arteries, and causes numerous other problems. Originally associated with diabetes, it is now thought to play a major role in the aging process and researchers hope to develop drugs that lock it.      　Despite years of research, scientists have identified only two  treatments that extend life without being aimed at specific diseases:  caloric restriction and hormone replacement therapy. Gerontologists have demonstrated in animals that reducing food consumption by about 30 percent below normal levels, while maintaining adequate levels of vitamins and minerals, can lead to a 40 to 50 percent increase in life span. However, maintaining such a diet regimen, requires a massive exercise of willpower that is probably beyond the reach of most people. What the researchers are seeking is a drug that would allow people to eat any of the foods they want, but enable them to still get the beneficial effects of caloric restriction. But that may be 20 years away.   Hormone replacement therapy, in contrast, is much easier and, in terms of quality of life, more beneficial. Recent studies have shown that hormone replacement therapy, which replaces naturally decreasing amounts of such female hormones as estrogen, can help prevent a variety of ills as well as reduce wrinkling and keep teeth sound. On average, hormone replacement therapy increases life expectancy by about eight years, with the greatest benefits among women who have high risk factors for heart disease. Studies in males taking estrogen also show a sharply reduced risk of heart disease. For most men, however, the feminizing effects outweigh the potential benefits.