I agree. The holdback parents will often say things like “there isn’t an age requirement in high school so get used to it.” I would say that when boys enter HS most have reached puberty. In middle school, however, you have some holdback kids who are 160 pounds playing against some on age kids who are 80 pounds. You will still have a few huge on age kids, and age based isn’t a perfect system, but keeps it as level as possible. | 
