Here's something I found:
Q: What are the requirements for a child to sit in a booster seat—age and/or weight?
A: These laws vary from state to state. Some use upper age limits—from 5 to 8 years old. Others weight limits—from 30 to 80 pounds. Some even use height limits—up to 4'9" tall. Some laws use a combination of these factors.
At its most simple, though, a booster seat should be used once your child has outgrown the upper weight limits of the forward-facing seat (which often happens around 40 pounds). He should use the booster seat until a seat belt fits him properly, which means that he can sit with his back against the back of the seat with his legs bending comfortably over the edge of the seat. The lap belt should fit snugly against his upper thighs or lower pelvis, while the shoulder belt comes across his collar bone (not across his shoulder or neck).
