Arbitrary (but required I suppose) age cut-offs are found in all sports. Early on it makes sense to have kids play among kids with same abilities. But as Malcolm Gladwell points out so well (book Outliers) in relation to Canadian hockey, older (and hence physically more mature) kids in the same year/grade/age group will be pushed along to better and more elite teams---an unforeseen advantage for being born just after the cut off date. I don't think that plays as much in the U.S., especially in baseball.