It depends on the pediatric dentist. Some states you are supposed to dismiss before 21. Most keep them for a few reasons...easy money if they are just on recall and patients and their parents hate to move on since they have been seen there for years. Ideally most have them move on in the teens, some when all primary teeth are gone. Trust me, I'd rather do restos on pedo teeth than adult teeth...MUCH easier! All of us bitch when we get adult dentistry in my residency, it's kind of pathetic in a way.
There is a thread in the pedo section of dentaltown if u want to read it from experienced pedos.