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?

But the more apt question is, What kind of world are they buying? A common thread running through Simpson's descriptions of young-old settlements in Arizona, Florida, Spain, and Japan is that aging residents are mere players on the stage of master developers. The inclusion of full-color brochures in the book is instructive'marketing, rather than jobs, family, friends, or long-held dreams, has unleashed this unusual migration.

A house is only the first thing they buy. To find as many middle-class seniors as live in The Villages, you would have to travel to a city the size of San Diego or Houston. As a result, this central-Florida community an hour northwest of Orlando is a stop on major concert tours. It's home to a tremendous diversity of medical establishments. And it's a dream for targeted advertising, which helps support developer-run newspapers and TV.

Self-segregation, meanwhile, mutes the shocks of age. The themed architecture does its best to feel old, from the generic casitas around Torremolinos on the Costa del Sol to the reproductions of 16th-century Dutch houses at Huis Ten Bosch, near Nagasaki, Japan. Isolated from the young and the poor, residents apparently feel spry and at ease. 'They are reborn'I think'when they come here,' says a senior official at The Villages.

It would be pleasurable enough to read Simpson's book for Studio Joost Grooten's dazzling maps and graphics, or for the parade of delightfully weird anecdotes from the world of the young-old: the tendency to peg the word 'golf' to streets, houses, and even landscape features on the Costa del Sol, for example, or the fact that America's most successful 20th-century retirement magnate, Del E. Webb, honed his city planning skills building Japanese internment camps during World War II.

But it's more serious than that. The young-old are a rapidly expanding class of global citizens. By 2050, the portion of the world's population over 60 will have grown from 12 to 20 percent. Their settlements portend our individual paths and the urban future at large. Let's hope, with their parades of borrowed forms, that they don't set an architectural example as well.