It’s the cutoff, not when it starts. In most European countries such as my own, you play with people from your age group, which is your calendar year. So, people born in January are playing with people born in December, nearly a year younger. This is especially beneficial for physical sports where a year difference can be very influential.

The September one can be explained because some countries have a September cutoff for academics/sports instead of January. At least, from my experience, UK and US have a September cutoff. In Spain, France, Italy Im sure the cutoff is in January, I’m not sure about the rest.