I turned my son at 2 years. he was 30 pounds then. he is 30 pounds still today (at 2.5). I wanted to wait longer, but we had just gone on a 20 hour drive with him rear faced and it didnt work out well, and we had to go on another 20 hour drive. (we moved down to california for a bit with my family, it didnt work out we moved back)
so for the next 20 hour drive, he had to be ff. it worked out very well. he sits in the middle either way also.