When President Carter's 1980 electoral defeat brought involuntary retirement, at age 56, from his position at the White House, he had no set plans for the future. According to this sprightly essay, he and wife Rosalynn, now in their 70s, have continued to lead full, active, productive lives because of their willingness to explore new commitments, their abiding refusal to be mentally dormant. Besides serving at the Carter Center in Atlanta--which they established to help negotiate peace agreements, to monitor elections in emerging democracies and to assist the elderly and mentally ill--the Carters are both university professors, and they roll up their sleeves to build at least one house per year for needy families. Further, claims the former president, they run three miles a day, take 15-mile cross-country bike rides and their sex life is ""more complete and enjoyable"" than ever. Carter dispenses sage advice on how older people can fashion an interesting and challenging life, strengthen interpersonal relations, maintain good health and face death with equanimity. While most of this counsel is not especially original and occasionally veers toward the platitudinous, he fleshes out his prescriptions with practical tips and pertinent examples of friends, relatives and associates who have remained productive. There are some remarkably intimate moments, as when Carter shares cathartic free verse that enabled him to face his ambivalent relationship with his father, or when he discusses the compromises that contributed to the success of his 52-year marriage. (Oct.) FYI: A volume in the Library of Contemporary Thought series.