I lived with my parents until I was 21-22. Up until 21, my parents said that while living in their house, curfew was 10:00 on school nights and 12:00 on the weekends. After 21, it got extended to 1:00am. Didn't matter how old. While I lived in their house, I had to play by their rules. They also said that if I did not maintain a minimum of 3.0 gpa in college, work at least 30 hours a week, and stay out of trouble, I was free to go out and get my own place any time I wanted it. Granted, while I was at their house, I was rent free, but had to self sustain my spending habbits.
In my background though, I was kind of a "troubled" kid and did get into some problems (sub 18 years old). This was my parent's way of giving me structure and discipline. Personally, I think it worked. Depending on how my kid is, I would have no problem emposing this type of system on them.