DD is 2 and already 18 kgs. We just bought the maxirider ahr and the shop assist rang safe n sound and you can use the harness fir over 18 kgs as long as it still fits ok. The maxirider has an adjustable head rest that moves up quite a fair way and the harness gies very high also. Once the harness us to small you use the anti sub clip with the normal car seat belt. Hth