We had a Dec 31 cutoff at the time DD started school. She is really tall and intellectually mature for her age. So far, it has worked out mostly well. She is more appropriately challenged and engaged in school than she would be if we'd help her back. We have had minor issues with sports - the cutoffs have not always matched well with the school cutoffs and she has been frustrated being on teams with mostly kids in lower grades (she is not great at sports, so can't justify playing up). I do worry about physical and emotional maturity as she gets closer to middle and high school, but I know we'd have been dealing with other problems for the last 5 years had we held her back!
ncat
mama to DD 12/04, DS1 11/08, and DS2 7/13