If youth is squandered on the young does it follow that old age is wasted on the octogenarian? Yes, insists Dr. Stonecypher, a California gerontologist -- as long as we persist in thinking of age 65 as a ""great divide,"" overprotect our old, tuck them into geriatric cradles and force them to exist on the periphery of life. Nothing leads faster to senility -- defined here as a psychological disease determined by two factors: personal resources and vulnerability to stress -- than ""sunset years"" taken up with busy work and spent in the exclusive company of the old where talk is apt to be demoralizingly about constipation, arthritis and the day's obituary notices. To build his case that functions which are exercised tend to persist, that those not tend to atrophy, Stonecypher cites, among others, such standard, irrefutable examples as Picasso and Casals who continued to be productive into their 90s not, as most of us assume, because they were geniuses (although one wonders) but rather because it never occurred to them not to. And it may sound like little more than semantics to read that aging doesn't cause death but that it only weakens body reserves and assists disease. Still, Stonecypher is provocative and readable, he has interesting new things to say about the physiology of aging and although not as profound as either de Beauvoir or Mannes, he may well -- ironically for just that reason -- reach a larger audience. And that, one feels strongly, is all to the good.