Older children should use a lap-shoulder seat belt in the back seat once they outgrow a booster seat, at least to age 8 and 4’9″ tall. They have not outgrown a booster seat until the seat belt fits correctly:
The shoulder strap should cross the center of the chest and rest on the shoulder (not the neck).
The lap belt should fit low and snug on the upper thighs (not the stomach).
The knees should bend at the edge of the vehicle seat when sitting all the way back.
Feet should rest flat on the floor.