The recommendation for use of a booster seat is that a child should weigh at least 40 pounds and be between the ages of 4 and 8. A child must also satisfy the height requirement such that he is at or below the height maximum of 4 feet and 9 inches.
Once a child is more than 8 years of age and also satisfies the weight and height recommendations for a booster seat, he can use a car seat belt if it fits him or her properly. However, the child should be seated in the back seat and properly buckled in.
At different ages, children will need different types of car seats that satisfy age and weight recommendations. For a child that is below the age of 2, the recommendation is a rear-facing car seat.