Kids need booster seats until they're eight years old or 4'9" tall. Booster seats work by raising the child up so adult safety belts fit snugly and correctly - flat across the collarbone and low over the hips, not across the neck or riding up over the stomach. Children who ride in poorly fitting safety belts can slide out of the belt system and be seriously hurt in a crash. So play it safe. Keep your kids in booster seats until they're eight years old or 4'9". Buckle Up. The Way to Go.