[ABCLAW]

If you're the oldest individual in your elementary class, you've got ~11 months more development than your worst-off competitors. You carry that age advantage throughout every level of education you go through, allowing you differential access to awards, accolades, special opportunities, etc.

When development evens out following full development, you're already past the majority of the educational sorting mechanism - you've been scoring higher and formed by your experiences as a high performing student throughout.

[dasil003]

But in athletics you stop getting training and don't go to the next level if your performance is sub-par, so the effect is magnified.