My oldest son turns 21 next month. He lives at home (for now) and works part time (35 hours at Starbucks) while he's trying to get a music career going. He doesn't go to school. Currently, he pays for his own entertainment, his gas (we own the car he uses), and gives us $75/mo, which pays his portion of a school loan, some of his cell phone, and some of his car insurance.
We've told him that when he turns 21, he'll have to pay a much greater share of his expenses. I'll continue to buy the food for the family, and we won't charge him rent as such (DH and I have already decided on this, so no need to address it). He stays on our health insurance, and it doesn't cost any more to cover him specifically. I've told him that he will be paying any medical co-pays and any resulting bills from medical appointments -- he's a healthy kid, so that shouldn't be a big deal. He'll pay for his prescriptions (1 at $10/month currently). I think he should pay for the majority of the insurance on the car he drives (DS17 also drives that car occasionally, so I won't make DS20 pay the whole bill), and all of his cell phone bill. Does that all sound reasonable so far?
Beyond that -- does he buy his own personal care products (shampoo, deodorant, etc)? All his own clothes and shoes? DH and I are planning to sit down with him before his birthday and have a financial planning meeting, so he knows this is coming. I want him to be saving to move out -- he had been planning to move this fall, but since he just recently, within the month, got a job after being out of work for 6 months, I don't think that will happen. He's more than welcome to stay for another year or so -- he's very pleasant to live with and it's good for his brother to have him here (but DS17 goes away to college in a year as well). Anything I've missed?