I lived with my mother (and my son) from 18-22 or 23. Her arrangement was that as long as I was in school, I could live there rent free, but leaving school was my signal that I was “grown up” now, and rent would be paid. I think it was $300 or so a month, less than half what my first apartment in the same city cost. I think it probably was a good motivator to keep in school, and as a teacher, school was always her priority. Up until recently, I shared her notion. But I’m beginning to think differently for several reasons, the main one being the crappy state of the economy right now.
The other is that my son, while just a lovely, lovely person, is also a bit immature and suffers from depression and anxiety which makes his daily life a struggle. He’s 17, and really panicking over deciding what he wants to do with his life, if/where he wants to continue college (he’s in an Associate’s program right now), career, etc. The idea that he might have to move out in a year terrifies him (and me, too).
When talking about this with my partner one day, he (partner) talked me around to his way of thinking. We decided that family is family, and as long as my son is willing to be a good roommate (and we talked at some length about that - namely, respect our space and time, call when you’re going to be late, do chores as needed), he’s got a home here as long as he wants it. True, we may move around a lot, his bed may one day be a couch or the pull down of an RV, but he’ll always be welcome under my roof without rent.
My daughter, the precocious little princess? I’m gonna start charging her rent next week. It can come out of her milk money. 
Seriously, I think you need to consider your family dynamics first, and the individual needs of your family members next. What’s right for one kid is wrong for another, just like every other parenting decision under the sun.
ETA: Too early for grammar…