Laura Carstensen's favorite lecture of the year is the one when she tells undergrads at Stanford University that—contrary to what they've probably been told and believe—these are not the best years of their lives. Those come much, much later. “I look out at a sea of 100 students sitting in a classroom, and I love the look on their faces. It’s just such relief,” says Carstensen, a professor of psychology and founding director of the Stanford Center on Longevity. “I can visibly see it, because