Arguably, birth date and astrology are only tangentially related. Iirc I looked into it and there was little empirical evidence for 12 groups, it is more like 4 groups spring/summer/fall/winter.
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.