I think, Snags, that Fruitsalad's question relates to the other end of the age range. (i.e. when can a youngster progress from a baby seat to a child seat).
If the car has front passenger air bag the child should not be in the front on a booster seat. For a child to sit on any seat without a booster seat their knees should be over the edge of the seat with their bottom against the back of the seat