Perhaps it is one of the happier side-effects of a youth-centered culture that middle age which used to begin at 40 in his father's best-best-selling book can now be postponed to 50. Whether or not this cheerfully realistic approach to growing old and staying young at the same time will follow commercially in his father's giant footsteps is hard to say, but it is certainly designed for the same market. Always a ""deference book,"" with some humor (intended) some seriousness (better achieved), this views what is gained at the mid-century mark (judgement, knowledge, etc.), what remains- considerable enjoyment, the problems of working versus retiring, money and planning for security in later years, the periods of energy and productivity which will continue but vary with others which are slacker, etc. etc. Mr. Pitkin's homely philosophy is buttressed with innumerable case histories (real and otherwise) and he views life with a wide-angled lens and never permits bifocals to blur the perspective ahead-- a good one.