Group 2-3 [15-36kg] seats are booster seats intended for children who are able to sit still the whole ride, because the adult seatbelt is used to restrain the child. The child using such seat should therefore be at last 4 years old, preferably older.
A group 0+ seat is outgrown when either the child reaches 13kg of weight, or the top of her head is in line with the top of the shell of the seat, whichever comes first. After that another larger seat with an integrated harness is needed; a group 1 [9-18kg] or group 1-2 [9-25kg] rear facing seat would be preferable because rear facing is safer.