Booster Seats. After age 4 and 40+ pounds, children can ride in a booster seat with the adult lap and shoulder belt until the adult safety belt will fit them properly (usually when the child is 4'9 tall, 10 12 years old). MUST have a lap/shoulder belt to use a booster seat.Phase 4.fter age 4 and 40+ pounds, children can ride in a booster seat with the adult lap and shoulder belt until the adult safety belt will fit them properly (usually when the child is 4'9 tall, 10 12 years old). MUST have a lap/shoulder belt to use a booster seat. Phase 4.