Kids born July - September are on the cusp of starting school. Parents will hold the kid back so they are 8-11 months older than their classmates so they have an academic advantage in maturity and an athletic advantage in size and growth. This was asked of us by other parents if we were going to hold her back. We didnt. She was the youngest in every grade and sports team growing up.
Its mostly in affluent (not really us) communities. Primarily with males to make them LEADERS. I remember seeing a story about a woman in Houston whose own daycare was shocked she was sending her kid to kindergarten as scheduled. Chicago schools shut it down completely, if your kid can go to school they do (unless you go to a private school).
Cooper Flagg rescheduled UP a year because the next college basketball recruiting class was seen as stronger than when he was eligible. Hes 1/2 way through his rookie NBA season at 19. He turns 20 in December so hes 19 and 2 months right now.
This all came from Malcom Gladwells 2008 book Outliers. He did include a study showing slightly older kids doing better in math and science but what really caught on was his analysis of Canadian hockey players. At the time , every Canadian in the Hockey Hall of Fame was born July - September (I think). Gretzky Lemieux etc. hockey drafted after the 7th or 8th grade. The kids basically go into a minor league system at clubs often staying at host homes. They get better coaching, equipment/facilities, and competition. The have a big advantage over kids left to public school and local club programs. Ive heard Gladwell say that he had no idea people would take that out of his book.