In her funny and wistful new book, Reeve Lindbergh contemplates entering a new stage in life, turning sixty, the period her mother, Anne Morrow Lindbergh, once described as "the youth of old age." It is a time of life, she writes, that produces some unexpected surprises. Age brings loss, but also love; disaster, but also delight. The second-graders Reeve taught many years ago are now middle-aged; her own children grow, marry, have children...