Apparently in the States they must rear face until they are 12 months but 2 years is recommended. I think to myself......wouldn't it get a bit squashy for their legs?
My slightly-taller-than-average 19-month-old is still rearward facing, and she's fine. Her legs do bend a little, she can't straighten them completely out, but I'd rather she break her leg/s in an accident than lose her life.