By Deane Simpson. Lars M'ller, March 2015, 384 pages, $50. In the fastest-growing city in America, the birth rate is less than half the U.S. average. Most of us aren't allowed to live there. It's The Villages, a Florida retirement community whose population has more than doubled since 2010 and now stands at 114,000. Ninety-eight percent white, 80 percent married, and 86 percent between the ages of 60 and 85, The Villages is uniquely homogeneous, banal, and bizarre by turns. America's largest gerontopolis is the most developed of four case studies in Young-Old: Urban Utopias of an Aging Society, an