Our seat was also up to 12Kg but we changed DS at sometime between 12-15 months. Can't remember his weight when we changed him but he's not 11kg yet and is 19 months. He was complaining a lot about being rear facing and his legs were also touching the back of the seat which is why we changed him. He is also quite an escape artist and wriggles out of the over the shoulder straps so is better that I can see what he is up to and pull over and restrap him in if necessary.