A good indicator if a child is ready to be out of a booster is to have them sit with their back fully against the seat. If their knees hang cofortably over the edge, then they are big enough for the seat belt. If they have to slouch to get their knees comfortably over the end of the seat, they still need a booster. When they slouch like that, the seat belt will not be postioned properly and they can have severe internal damages in an accident or even a fast stop. It happened to my cousins daughter and it was something you never want your child to go through. She was in the hospital for quite a while after that accident.