Though safety-seat laws vary from state to state, all require that children under age 3 ride in a safety seat. Experts are unequivocal in their recommendations for safe riding beyond that age:
• Your child should ride in a safety seat with a five-point harness until he weighs at least 40 pounds, or until his shoulders no longer fit under the harness straps. You can use a convertible rear- and forward-facing car seat until your child hits 40 pounds, or the harness system of a car-and-booster-seat combo from as little as 20 pounds up to 40 pounds.
• Your child should ride in a booster seat from the time he weighs 40 pounds and is at least 3 years old until he's 4 feet 9 inches tall and at least 8 years old.