When I was 12 I ran 5:05 mile in jr. high and a 28:50, 5 mile. I am sure I could have done similar 6 minute pace races and 5:00-ish mile times at age 10, or 11, but my only other timed races were some 10 and 13 milers with my Dad. I know I was doing training runs with my dad at 6-7 minute pace but don't have any earlier timed records. My fitness came from insane swim workouts twice a day, caddying on a golf course all summer long, and biking everywhere, everyday. My brother, 1 year older, was even faster than me, but was not interested in races. Two points:
1. The only reason I think I could beat all others my age, at the time (1970s), was I think there was nobody my age running like that. And yes, Dad had no clue that this was probably not good for me.
2. At that time, there was no shortage of guys slightly older 13-15 years old, who could run much faster.
I think if people put a clock to all the kids (6-10) out there playing soccer, swimming etc., we would see some similar times.