"It is time to be old, To take in sail.
-- Ralph Waldo Emerson
We age, of course, because . . . not so easy to answer that, is it? Well, we might say that we age simply because, as Emerson said, it is time. Everybody and everything ages, don't they? People, animals, trees, even stones age. If we continue to exist, we eventually grow old. All it takes is the passage of time. An African Bushman once told a visiting anthropologist who had asked him what made him old: "Well, you know, what happens is that time passes. The rain comes, and then the dry season, then it rains again. That's what moves you.' 1 It is more complicated than that. The phenome-