Height and weight more important than age.
Never buy a child car seat that is too big, make sure it is appropriate for your child’s height (i-Size) or weight (ECE R44/04 ) from the first day of use. The appropriate category is stated on the orange ECE approval often placed at the bottom of the car seat. Age categories are also often stated, but because one child can be heavier than the other, the age group should be considered primarily an indication.
It is safest to buy a car seat to suit your child’s height or weight as they grow, rather than buy a car seat that covers the whole weight range (using the same car seat from birth to 3½ years, or longer).