Irrepressible writer-comedian Carl Reiner, who shows no signs of slowing down at 94, tracks down celebrated nonagenarians, and a few others over 100, to show how the twilight years can truly be the happiest and most rewarding. Among those who share their insights into what it takes to be vital and productive in older age are Mel Brooks, Dick Van Dyke, Kirk Douglas, Norman Lear, Betty White and Tony Bennett.