My parents have a rule that as long as you're in school, you don't have to pay rent. As soon as you're out of school though you have to pay up. That's their way of motivating us to move out of the house. My mom was originally going to charge my older sister $500/month, but she and my dad talked her down to $300/month. Can't say it wasn't effective. My older sister moved out a month or two after graduating college. I moved out the week after. My older brother would probably still be at home if it weren't for that rule.

If we really needed a place to stay and couldn't afford that rent, I'm sure my parents would be accommodating. They just want to push us to be independent.