Redshirting all boys, i.e. having them start school at age 6, not 5, is becoming more recommended, not for sports but for maturity in general, e.g.:
https://www.theatlantic.com/ma...-redshirting/671238/
(not sure whether that's behind a paywall). If you google "boys start school 6", you find many studies saying that wealthier families routinely do this, poorer ones less so.