Well they usually recommend that at 18kgs BUT all carseats, esp the SnS brand are tested at 22kgs for 5 point and 32kgs for booster capacity. So you have a bit of leeway. Britax have stated that a child of 20kgs using 5 point will not compromise the safety of the seat BUT they cannot legally recommend it. Ridiculous.

I wish we would adapt to follow Scandinavian car restraint laws. Rear facing til 4 years old and they have booster for 8-32kgs that have 5 point harness AND rear face!