XP

An earlier birthday directly correlates to a higher chance of becoming a professional athlete. This has to do with how people born in January are still competing in the same age group as people born in December resulting in an up to basically a complete years worth of advantage (age groups are only based off of the year you were born in).

This inconspicuous advantage will cause your teammates to view you as a better and more talented player and will increase your liking of a certain sport. If your coach thinks you're a better player, he'll try harder to push you. You'll have more time on the field. You'll play more games. You'll participate in more tournaments.

The list just goes on and on, each effect snowballing in its own right. More play time = more experience = faster growth = etc. What started as a major advantage will only grow bigger.