Many households of the Boomer, X, and Y generations are living together as a result of the recession. Homebuilders are taking their cue from the trend, seeking to diversify their future master-planned communities. Age-diverse neighborhoods tend to evolve naturally over time. Rather than leaving it to chance, builders are creating housing developments that they hope will appeal to singles, younger households with and without children, retirees, and multi-generational families. Amy Hoak of the Wall Street Journal reports on the buzz.