In a series of generally sage essays, historian Smith (Democracy on Trial, p. 767, etc.) forsakes public chronicles for the private kind as he describes the passage toward a destination from whose bourne no traveler is likely to return. The professor is now in his eighth decade and sufficiently long in the tooth to provide a geriatric study direct from the horse's mouth, as it were. He is sprightly as well as wrinkled, clever as well as creaky, querulous, and crotchety. Age, he says, ``is a pain in the ass.'' He is a sagacious old guy, but do not call him a ``senior citizen''who wants that nice-nellyism? Do not offer him a demeaning ``senior discount,'' either. Don't take him for a golf outing or on a cruise. (Tennis and trout fishing would be just fine, though.) Worst of all for those who have something to contribute and a full complement of their marbles is talk about retirement. It is anathema to Smith, who calls for a constitutional amendment to prevent enforced retirement. That's ageism, which according to Smith is really ``prejudice against the future of the self.'' Occasionally veering toward Hallmarkism, as in a piece on the admitted charms of grandmothers, the wise old prof never wanders far from truth. His guide ranges from geezer jokes and old health to thoughts on elder sex (not a bad idea) and death (expect it). The text surely derives more from the primeval need to edify future generations than from the need to offer advice to fellow oldsters. A bit of clear thinking on some age-old questions about old age.