Agism is the notion that people cease to be people, cease to be the same people or become people of a distinct and inferior kind, by virtue of having lived a specified number of years."" Thus gerontologist Comfort in a classy, brassy blast before Gray Panther barricades. This encyclopedia for the elderly begins with exploitative ""Agism"" and proceeds through such entries as: Doctors (""be suspicious if you are told ill health is what you can expect for your age""); Exercise (""the best exercise is work""); Masturbation (""normal and healthy at any age""); Nursing Homes (""You wouldn't voluntarily put a relative in Belsen or go there yourself""); Pulling Rank (""use it""); and Youth (""join forces against the oppressors in the middie""). Although the old grew up with society's prejudices, that's no reason, insists Comfort, for them to ""obligingly drown themselves."" The author dwells on all the good things about old age including the aerial view of one's own life: ""by sixty-five you begin to realize that you have spent too much time in Disneyland.' Often the fully realized older person ""doesn't retire. . . but stays 'in' the world raising radical hell for the tights of others."" Along with upbeat, if cheerily simplified medical advice, and some quieter discussions of death and bereavement, there are the inevitable success stories of famous and ordinary elderlys who hit their strides in later life. Forget about those brain cells dropping away--the number is relatively insignificant--awake and live. With attractive drawings of seniors--from Mao to Bob Hope, Ghandi to the two Dukes--it's the best tonic for old persons since the spike was removed from Lydia Pinkham's.