For Claireybear (and for Fuggy, if still confused):
To clarify the position regarding the relevant criteria:
The appropriate type of restraint (i.e. baby seat, child seat, booster seat or booster cushion) is, as I've stated, defined by the child's WEIGHT. (Details are given in Section 3 of the link I gave).
However, the requirement to use any type of restraint (other than the adult seat belt) is determined by the child's HEIGHT. (Once a child reaches 135cm, there is no longer any requirement for the use of a special child restraint). Most children will reach this height around 9 years of age. If a child is very small for their age, then their AGE finally becomes the determining criterion. (At 12 years of age, irrespective of either weight or height, a child is no longer required to use any form of restraint other than the adult seat belt). The final relevant age is 14 years. This is when legal responsibility for ensuring that a seat belt is worn passes from the driver to the young passenger.
Chris