I think it was a joke. The statistical factor is actually age: the older kids perform better because they're more grown up (physically bigger, mentally more developed).

So if you set a cut off point at September, for example, kids born just after September: October, November, December, will be the biggest ones.

For example for an age group covering up to 11 years old in September 2024, the kids that are 10 years and 9/10/11 months old (so born in October, November, December 2013) will statistically be the best players of their group.