I can't remember even keeping score till I was 11-12, maybe a couple 9-hole rounds in the 60-65 range.
My freshmen year of HS in 1994, I played #7 on my team (of
and won every 9-hole match I played with a scoring average of 51.5 or so. Thanks to Tiger, you have to be much better than that to make the HS team today!
We used to play interclub tournaments with the local clubs, and my yearly scores at Essex County Club were 124-89-76 at one point. I was not good enough young enough to play high-level junior golf, but during my HS/College ages of 14-22, my handicap went 28-22-12-6-4-2-1-0.
I do not remember breaking 100 the first time, but I vividly remember breaking 90 in Hawaii with an 85 at Kauai Lagoons (the NLE Mokihana course...9 holes may remain) when I was 15, and I holed a 50-y pitch at my home course to break 80 (78!) for the first time the next summer. My dad offered a $100 prize for the first completely legit round that I beat him (he's a longtime 10ish-capper), which happened around that time too.