@levisan those communities are designed for older people: one storey buildings with minimal flat thresholds on the doors, community services to cover what old people increasingly cannot (lawn, snow clearing, exterior maintenance of the home).
Excluding younger people limits parties, crazy drivers, and people out of step with the rhythm of life of retirees.
No booming house/car stereos, no skateboarders / scooter riders to wipe out driving... and obvious indications that someone doesn't belong in the neighborhood.