They're becoming more common lately, particularly as adolescence is extended and adulthood pushed further and further away.
Bum around for a year or two before college. If you're well-heeled, go backpack across Europe. Go out and party, sow your wild oats, start college at 20 or so.
Might as well. It's not like there are awesome careers out there waiting for you to graduate.
Someone posted a few days ago about a northern state's law having your kids on your insurance until they're 28 or something. That's ridiculous. Can you imagine a World War or Depression with these grown snowflakes hiding out in mama's "safe space" basement with their blankies?