Think your child is ready to ride in a booster seat? NHTSA recommends children should continue to travel in a car seat until they reach the maximum forward-facing height or weight, which might not be until age 8 or older.
When you and your child are ready to transition to a booster, choose one that properly positions both the lap and shoulder belts on your child. Learn more about booster seats: https://www.safekids.org/safetytips/field_risks/booster-seat
Original source can be found here.