When David Harrison began studying aging, he had yet to experience its effects. In his late twenties, he was fit, healthy and impervious to harsh New England winters; on all but the coldest days, he'd strap on cross-country skis and head for the Maine hills. Now, though, at age 67, he sees in himself the progressive decline he has observed in the mice and other animals of his research. A decade ago, doctors removed a prostate tumor before the cancer spread to his bones, but other problems have accumulated. Some, such as high blood pressure and high cholesterol, are easily treated, but they're accompanied by elevated insulin levels and weight gain - a constellation of disorders called 'metabolic syndrome," which often precedes heart disease, diabetes and stroke. Dieting, exercise and drugs only slow its progression. And he now spends winters in North Carolina, where the milder climate seems to help him dodge another apparent symptom of his advancing years: chest colds that linger for weeks.