They still need to be within the limits of the car seat. Pretty sure all the car seats available here are only rear facing til 12kg. Length of their legs isn't a factor, but fitting in the restraints and their weight is We stayed rear facing for a long time quite easily because my kids were fairly lightweight. DS still fits in the 5 point harness of his maxi rider at the age of 5! We haven't converted it to booster seat yet We'll keep this next bub rear facing as long as possible too. It's just too dangerous to have them facing forwards