Children who are taller or weigh more than the limit for a forward-facing car seat should switch to a belt-positioning booster seat, according to the American Academy of Pediatrics. It's important to check your car seat owner's manual for the seat's height or weight limit.
Booster seats are designed to raise children up on the car seat so that the lap and shoulder belts fit correctly. This happens when their ears are level with the top of the back of the safety seat, and their shoulders are above the top strap slots. Children should use a belt-positioning booster seat until the vehicle seat belt fits correctly. Typically this is when they have reached 4 feet 9 inches in height and are 8 through 12 years of age.
Booster seats should always be placed in the back seat of the vehicle. All children younger than 13 years should ride in the back seat.