My son has a Dec bday so we didn't have to face this decision, but I wanted to tell you what some of the parents of older children at one of my CF support meanings said that needed to be taken into consideration. Children with CF are usually "late bloomers" meaning for the most part they tend to hit puberty later so it can be more difficult for a CF child that it already young for their grade. IF my son was even a few months younger, we would have held him back since both my husband and I both were "late bloomers."