I'm pretty sure I've seen a thread on this topic, but I can't find it now. My 37 month old is only 35" tall and 28 pounds and uses the second from the top slot in his Roundabout 50, rearfacing. I expect he'll fit rear facing in the seat for quite a while. I might get him a radian when my new LO outgrows the infant seat. Would it make sense to keep him rear facing as long as he fits, probably till 5 or 6 years old, and then go directly to a booster? Then again, I don't know if he'll complain about being RFing at some point, but for now, even though all his friends are FFing, he loves his seat and enjoys being able to see the kids behind him.