For us, the decision to start our kids (with late July birthdates no less) to school at the age of 5 had to do with them being ready. We did not look into what would make them a better athlete. Looking back, it was the right decision for OUR kids. Yes, they have had to compete both academically and athletically with kids who are 1 to 2 years older than them everyday and every year. But again, if you did not know when their birthdate was and how old they are, you would never know. Are they competitive? Yes. Would they have been better if they were in a class below them? Maybe. Sometimes it is the people who are your classmates and friends that make you a better athlete. Not whether you can dominate in a class or not.
The whole point of this topic is not whether you should start your child off to school at an early age. It is if there is a problem with a 20 year old wrestling in high school. Or if you have a problem with a 15 year old wrestling in 8th grade. As a parent, I do not care about the opponents age as much as I care about the size (especially with football starting). My son has competed against some very "old" 8th graders when he was still 13. It hasn't hurt him. In fact, his toughest opponent (in all JR High NCKL sports) was in track and he was only 13 too.