A booster seat is used for children who weigh at least 18 kgs (40 lbs.). Boosters will have maximum weight limits of 36 kgs. (80 lbs.), 45.5 kgs. (100 lbs.), or even 54.5 kgs. (120 lbs.). Some boosters will have minimum age suggestions in their manuals (i.e. 4 years old). If a child is young but heavy, a forward-facing seat with a high weight limit would be the best option.
Children should stay in their booster seats until they fit the adult seat belt properly. Most children should remain in booster seats until they are between the ages of 8-12 years.