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 intriguing and vital new book by architect Deane Simpson, who teaches at the Royal Danish Academy School of Architecture in Copenhagen and at BAS Bergen in Norway. His subject is the 'young-old,' a budding, first-world class of seniors settling the newly opened land between the end of responsibility and the beginning of dependency. 'Millions of retirees around the world consciously choose to spatially secede from the rest of society to achieve the ideal lifestyle,' Simpson writes. You might ask, What kind of world are they building?