On being older when in school. Not always holding a kid back.
Military family here. Five different schools in two years at one time, in four different countries. US Military schools as well as normal, in country attendance in others.
When I was a HS Junior in a US State I had already takan all but one class needed for Graduation but this was a State that did not do 'early graduation'. If we were at the last posted location I would have been done that year. Ended up finishing the Senior year with all electives - do I too Auto Mechanics & shop & typing along with what advanced classes in Chemistry the school offered. 1960's without the AP and pre-placement stuff available later to our kids.
Now I would have been in college early. Not then. A different State - different rules and opportunities.
Athletics? Having entered AAU tournaments & European tournaments - made me ineligible for HS wrestling, baseball and Cross Country in this State my final two years. Pretty unfair then and since I never went back after leaving I don't know what they do now.
Some kids are early, some later for whatever reasons. Nobody seems to care if it is Physics, Chemistry, Music, Art or Theatre but go apeshiz with Athletics.