SOONER OR LATER, all of us are going to be old; and because of increased longevity, there are more Americans over 65 every year. But increasingly, fewer Americans are content to sit back, rock their rocking chairs, and watch the Polident fizz around their upper plate. A large number of books has arisen to serve the needs of senior citizens - and of some youngers ones who are thinking now of what are hypocritically called the golden years.

If good health is really the most important thing, it is worth taking a look at two similar volumes which want to prepare you for a healthy old age or, failing that, to help you retain what you have left. Welcome to the Middle Years, by Robert Taylor, M.D. (Acropolis, $10) is a good if rather brisk guide to proper diet, the future of your skin and hair, menopause, hysterectomy and potency, sensible use of drugs including alcohol, exercise, heart attacks, high blood pressure and a host of other vital concerns. Dr. Kugler's Seven Keys to a Longer Life, (Stein & Day, $8.95) covers the same territory, but with a slightly different emphasis on environmental dangers and compensatory remedies. (For example, if you can't quit smoking, take more vitamin C - each cigarette destroys about 20 mg of the sunshine vitamin.)

For those who are contemplating a life-change of some kind, there is Herbert B. Livesey's Second Chance (Lippincott, $8.95) which details numerous case histories of those who decided to change careers, and the prices they had to pay to do so. There is a strong emphasis on self-employment, which is increasingly popular among retired or semiretired persons. And if you just want to learn more, The Lifelong Learner, by Ronald Gross (Simon & Schuster, $8.95) can help you to matriculate in the "invisible university" - the plethora of educational institutions and programs available to the adult student in an age of declining youth enrollment.

Two books debate the question of how to keep the tarnish off the golden years. Hanging In There, by Peter Schwed (Houghton Mifflin, $6.95), is a short admonition to keep active, with plentiful suggestions on how to do it. Some more militant Grey Panthers might object to Schwed's remarks on the joys of stamp-collecting, knitting and basket-weaving, but it is hard to fault his essential good humor. The Retirement Handbook, by Joseph C. Buckley, revised by Henry Schmidt (Harpet & Row, $9.50) is now in its sixth edition, and treats a wide range of questions the prospective retiree must face, from the obvious ("Confine yourself to easy-to-digest foods") to the technically essential ("Taxpayers over 65 can now sell a home worth $35,000 without incurring any capital gains tax").

Finally, Estelle Fuchs's The Second Season: Life, Love and Sex - Women in the Middle Year (Anchor/Doubleday, $8.95) explores the many changing aspects of a woman's life in middle age. Dr. Fuchs is an anthropologist, and it shows in her treatment of the subject, which is scholarly and wide-ranging. Of particular interest is her long discussion of menopause: how it is treated in other cultures, and how we perceive it in ours.