How old are we, those of us who belong to the postwar era? By many measures, both evolutionary and cultural, we are older than ever. But we are also getting startlingly younger--younger in looks, attire, behavior, mentality, desires. We belong, Robert Harrison says, to an age of juvenescence. Juvenescence is about the ways in which the spirits of youth and age have coexisted and shaped each other, both in individuals and culture, from the time of antiquity to the present. It is also a book that asks what it means for the future when youth gains the upper hand to the unprecedented degree it has today. Our way of aging, Harrison argues, resembles thethe scientific concept of neoteny--the retention of immature characteristics into adulthood. We mature, but with a still tenacious youthfulness, driving drives toward innovation rather than reflection, genius rather than wisdom. At its best, human maturity has its source in the youth it brings to fruition. And yet our protracted youth, Harrison suggests, is a luxury that can be supported only by our elders and the institutions they build. Although Harrison believes, echoing Stephen Jay Gould, that "our genius as a species lies in our collective reluctance to grow up," he argues that we are today in a phase of radical juvenalization that allows no space for the kind of wisdom that builds upon the past.