Surveys of automotive child restraint use in various countries have repeatedly shown that many parents do not select the correct type of restraint for the child and lack knowledge about correct restraint selection. Advice to parents is based on the dimensions of the child, usually weight, but such prescriptions may be difficult for parents to remember, and parents often do not know the weight of their children. The child's age might be preferable for promotion and regulation because parents know it. Using a dataset of the distribution of children’s weights at one-month intervals of age, and assuming that all children change from one restraint device to another at a particular age, we demonstrate the trade-off between the number of children too large for the smaller device and the number too small for the larger device. This is used to suggest an optimum transition age. The regulatory jurisdictions of Australasia, Europe and the United States of America are compared. The analysis shows that in Australasia, where there are currently significant overlaps in the weight ranges of each type of restraint, recommendations to make restraint transitions at 6 months and 4 years of age would mean that about 10% of all children under the age of 8 would be in a restraint unsuited to their weight. Corresponding figures for the European and United States Standards are 6% and 16%. Instead of battling to get parents to use child’s weight as the criterion for restraint selection, it might be better to promote exact ages as the transition criteria, and to write the Standards for child restraints on the basis that this will happen.