My ds has a Dec bday and the cutoff here is Sept 2 so thank goodness we won't have to make that tough decision (we will be in 1st grade next year). I was a Sept baby when the cutoff was Oct 1st. I so wished my parents had held me back. I was very smart (they actually wanted to move me ahead another year and thank goodness my mom said "no") but not as mature socially. It was tough for me. I also was a "later bloomer" (hit puberty late - maybe due to being a CF carrier but who knows) and that made it tough for me too. I think that every child is different, but I think if he is ready academically AND socially, then I would send him on. I just wouldn't forget that most CFrs do hit puberty later than average and that could be tough on a teenager. This is only my opinion and only you know what is best for your child.