Agreed. Why do people freak out about this? Now, if a 20-something won't get a job, won't help with house chores, etc, yes, that punk needs to be kicked out. But if he works, pays his own bills, and pulls his own weight, why not? Instead of wasting money on renting his own place, bank the money for a future matrimonial nest egg. That's what I did.
Of course, I'm old-fashioned. If you're living an upright single life, there's nothing to hide from your parents; but if you are engaging in premarital sex, then of course you need your own place for privacy. Otherwise, there's no need to move out til marriage.
You hit on something. If kids could sock more money away for retirement while they are in their early 20s, that would make a huge difference in the amount of money they could have for their retirement, than if they started saving in their late 20s/early 30s.