The AAP recommends
rear facing to the limits of the carseat. The biggest improvement in safety is seen when the child is under age 2. A two year old child is 4-5 times less likely to die or be seriously injured in a crash when properly restrained in a
rear facing carseat.
You need to keep an eye on both the weight and height limits. A
rf carseat is outgrown when child reaches the weight limit or by height when there is less than 1" of hard shell above the child's head.