Answer:
A booster should be used until EITHER: the adult lap/sash belt fits the child without the need for the booster OR the child's eye level when seated on the booster is the same as the top of the booster (or the car's seat back or head rest - ie there is no support for the head) OR the child weighs 26 kilos. To tell whether the adult seat belt fits a child, the child needs to be placed in the chosen seating position and check that the sash part of the seat belt sits flat on the shoulder and does not come in contact with the child's face or neck. Lap/sash belts offer far greater protection than lap only belts. Because children are far more fragile than adults, putting them in a position that has a lap belt only should be avoided. Whenever possible a lap/sash belt should be used with a child. When fitted correctly a lap/sash seat belt does not come into contact with the face or neck.