Why do we have to go over this a million times? Age cut-off for kindergarten and following is typically September 1, right about the beginning of the school year. Typical Americans go to kindergarten when they are five years old. So, they are born from, say, September 1, 2008 to August 30, 2009. That means that all high school freshmen now who were not held back or skipped a grade would be 15 years old or about to turn 15 this summer. About one-third of them will turn sixteen this year, from September through December. Two-thirds will turn sixteen next year, from January through August. Then there are about 10% of kids, mostly boys, who were held back and entered kindergarten at age six, in some cases because boys tend to be ready for kindergarten later than girls (and grades and behavior charts show this) and in others for athletic purposes later on. If he turns 16 prior to September 1, he is the age of a sophomore, though not by much. If he was born in August and is therefore a few weeks older than the oldest typical kid in his class, then I can understand why they would have done that, because my son is among the youngest in his class and as a result has a disadvantage in physical and mental development relative to kids as much as 11 months older than him with whom he is competing at school and in sports (and doing very well but in sports there is a definite disadvantage).
(For those considering skipping kids, as we did our daughter, I would advise against it because even if the kid is academically more than ready to be a year, two years, three years or more ahead, the social and emotional, as well as the physical, disadvantages make it not worth it by high school.)