A.F.
This is the site I refered to when I thought my daughter was responsible enough for a booster seat. I know she can handle keeping the seat belt on in the right place while driving, her brother however, will probably be in a 5 point harness until he doesn't fit anymore. I can see him pushing the release, or crawling out, and joining me in the front seat, or crawling around in the back. Hope this helps.
http://www.infant-car-seats.com/states/car-seats-laws-Rho...