I've seen dads so it around 8th grade for sports. I've seen other parents send their kids to private schools and when they migrate to public, they start a year behind. Others use the homeschooling excuse or whatever. I say do whatever floats your boat, we "held" our son back in kindergarten as my wife was an educator and my son was a bit unfocused and she conferred with many who advised her that would have the least effect vs later in life. We had zero idea he would play baseball or any sport really. He was a young 2014 then and older 2015 afterwards which i guess could have helped him in baseball but he was still smaller than his teammates from about 12U thru HS. It was about maturity in the classroom back then.