Seat belts are made for adults. Children should stay in a booster seat until adult seat belts fit correctly, typically when children reach about 4 feet 9 inches in height and are 8 through 12 years of age. Most children will not fit in a seat belt alone until 10 to 11 years of age.
When children are old enough and large enough to use the vehicle seat belt alone, they should always use lap and shoulder seat belts for the best protection. All children younger than 13 years should ride in the back seat.
Using a Seat Belt:
An adult seat belt fits correctly when:
The shoulder belt lies across the middle of the chest and shoulder, not the neck or throat.
The lap belt is low and snug across the upper thighs, not the belly.
Your child is tall enough to sit against the vehicle seat back with her knees bent over the edge of the seat without slouching and can comfortably stay in this position throughout the trip.
Other points to keep in mind when using seat belts include:
Make sure your child does not tuck the shoulder belt under her arm or behind her back. This leaves the upper body unprotected and adds extra slack to the seat belt system, putting your child at risk of severe injury in a crash or with sudden braking.
Never allow anyone to share seat belts. All passengers must have their own car seats or seat belts.
Common Question about Seat Belts:
I've seen products that say they can help make the seat belt fit better. Should we get one of these?
No, these products should not be used. They may actually interfere with proper seat belt fit by causing the lap belt to ride too high on the stomach or making the shoulder belt too loose. They can even damage the seat belt. This rule applies to car seats too; do not use extra products unless they came with the seat or are specifically approved by the seat manufacturer. These products are not covered by any federal safety standards, and the American Academy of Pediatrics (AAP) does not recommend they be used. As long as children are riding in the correct restraint for their size, they should not need to use additional devices.
Additional Information from HealthyChildren.org: