Generally speaking, a Group 0 or 0+ seat is for a child aged up to nine or 15 months (up to 10 or 13kg) respectively; a Group 1 seat is for children aged between nine months and four years (9-18kg); a Group 2 seat covers four- to six-year-olds (15-25kg); a Group 3 seat is for the under-12s (22-36kg).
Move up to the next group if the top of your child's head is above the top of the seat.
A child must use a seat until they are at least 135cm tall - if not, you could get a 30 fixed penalty or a fine of up to 500.
If you have been in a crash, you must replace each child seat in the car. They may have been severely weakened in the accident, so you'll be putting your child at risk.