Our daughter is pretty much on track to be the same size as yours probably, and I just figure, does my 7 or 8 year old really need to be rear facing? Many kids reach the limit far before that age, and a 7 or 8 year old would be much more physically developed than a 3 or 4 year old.
We JUST started forward facing our daughter last week, and she was 3 in October. We only did so because our second born is due on Feb. 14 and requires the spot behind me for the rear facing bucket seat. She is about 28lbs and 36in. I would prefer her to still be in the rear position but it won't work for our vehicle as dad is a tall, big guy. (6' and 190-200lbs).