An estimated 20 million young-adult Americans who left home have moved back with Mom and Dad. The reasons vary. Some could not afford the high cost of rental housing on their own. Some got into trouble with credit-card debt. Some have come back, in part, to help their aging kin. But others just wanted to save a few bucks to go back to college -- or simply realized it's a pretty good deal to live rent-free, with meals cooked by Mom, while they play in a garage band, dabble in acting, or contemplate what they want to do with their lives.

"Boomerang adults," they're called -- going away for awhile but circling back home.

So much for their parents' dreams of the empty nest -- quiet, unencumbered, free from worry about when Mary or Jimmy will be home safely with the car.

And now the boomerangers are knocking on grandma and grandpa's doors as well! If you peek at the tennis courts, swimming pools, or exercise classes at America's 1,300 so-called "active adult" luxury retirement communities, you'll see not just gray-haired men and women but also much younger adults in the prime of their lives.

The New York Times discovered that in many towns, once the old folks come up with the money to move into a pleasant condo community, younger relatives are free to join them.

What will it be today for these daughters and grandsons and nieces living at the gated retirement village? Let's see: aerobics, perhaps. Or golf. Or maybe that bird-watching hike. And what did you say is on the menu for dinner?

Wait, these young folks tell the Times, there are hardships. Living with elderly parents or grandparents can put a damper on your love life. But then, what young person's bachelor pad has central air conditioning, a washing machine and dryer down the hall, and a jet-spray Jacuzzi bathtub?