untill they pass the 5 step test.
The 5-Step Test
1. Does the child sit all the way back against the auto seat?
2. Do the child's knees bend comfortably at the edge of the auto seat?
3. Does the belt cross the shoulder between the neck and arm?
4. Is the lap belt as low as possible, touching the thighs?
5. Can the child stay seated like this for the whole trip? If you answered "no" to any of these questions, your child needs a booster seat
to make both the shoulder belt and the lap belt fit right for the best crash
protection. Your child will be more comfortable, too