Here in Australia you can only keep them rear facing until they reach the carseat's weight limit...for most seats that is either 9 or 12kg...then you have to turn them around.
Unfortunately you can't buy seats here that have a higher rear facing weight limit...hopefully as more people realise how much safer it is they will start to produce hight limit seats.

My DS is 15mths but only 6kg so he will be rear facing till at least two.