we were at a car seat check about 3 years back. They told us the min. age/weight for booster seats was 4 years OR 40lbs.
They no longer need to be in a booster seat when their feet can be firmly planted on the floor while their back is straight and up against the back of the seat. This was when the booster age increased to 6 years old. Eddie had just turned 6. The car seat checkers told us he didn't need to be in a booster seat because he was so tall.
http://www.health.state.ny.us/publications/3090.pdf