Hi, it depends on where in the world you are but a website I found confirms it should be on baby weight and not age necessarily. Here is some of the information:
Rearward-facing Baby Seats
Group 0: for babies up to 10 kgs (22 lbs) roughly from birth to 6-9 months, or
Group 0+: for babies up to 13kg (29lbs) roughly from birth to 12-15 months
Forward-facing child seat
Group 1: for children weighing 9-18 kgs (20-40 lbs) roughly from 9 months - 4 years.
Hope this helps