Like Cancer, another Silverstein title, this might seem a less than compelling subject for a children's book. But whatever its readers' motivation, they will find here another of the Silversteins' exemplary research reports. Every animal or plant species seems to have its own typical lifespan, they note; and though many of the physical and mental concomitants of growing old are not so much inherent, inevitable processes as the results of disuse or accumulated wear and tear, a limit of about a hundred to a hundred-fifteen years seems to be programmed into our genes. The recently celebrated long-lived populations such as the Hunzans and the Soviet Georgians were really just faking their ages, most researchers now believe, and all youth elixirs from the fabled fountain to yogurt and royal jelly have proved illusory--though recent studies with mice and rats, based on investigations of the probably many and varied causes of aging, have found a number of substances that stretch out their lifespans. The Silversteins don't consider how we will handle the population pressures if the dreams these studies encourage ever come true, but they end with a perhaps more realistic look at the plight of forced retirees. Illustrated with photos of such creative old people as Picasso, Frank Lloyd Wright, and Grandma Moses.