Fundamentally, you have to look at two factors: the child’s weight and height.
The main parameter is weight: the seats are classified into groups based on the weight of the children who can use them. A child should never use a child seat if they are more than the maximum weight indicated on the seat: the danger of the seat breaking in the event of an accident is too high.
You also have to take into account their height: the child’s head must rest safely and comfortably on the back of the seat. If the crown (or top of the child’s head) is higher than the top edge of the seat back, then it is time to change it and use a larger seat or one from a higher group.
Age is a less important parameter, especially since there can be considerable differences in weight and height between children of the same age. But always remember that until the baby is 1 year old and weighs at least 9 kg, they must travel in a rear-facing position.