Youngtown, Arizona, opened in 1954 and was the first development community to have a minimum age requirement (then 65) and to ban underage children as permanent residents. Developer Del Webb unveiled Sun City six years later. Adjacent to Youngtown, it offered modest homes abutting a golf course. In the ensuing decades, active adult communities have proliferated, including Harold Schwartz's "The Villages" in central Florida, today the nation's single...