Booster seats are for children who have outgrown convertible or toddler seats. Booster seats are for children who are 4 to eight 8 old, weigh 40 to 80 pounds, and are less than 4 feet, 9 inches in height. You can use a booster seat until an adult seat belt correctly fits the child without use of the booster seat. Always use both the shoulder and lap belts with a booster seat.