The point is not that younger children are dumber than the ones a couple of months older than them. The point is that at such a young age kids make tremendous developmental strides in a short amount of months. Hence the idea of dividing young kids up according to age. Actually, where I live, they do it in the boys school and the girls and it makes a huge difference. Obviously there are exceptions to every rule, but in general maturity and acquiring basic fundamental concepts are better achieved and received in classes divided by age.
Usually by second grade they mix the classes and you have all months in every grade.