I wish I had charted my velocity through the years, and it wasnt common to I have a gun on me until my Jr year of HS (we didnt have quite the number of showcases and resources available now), but going off approximate memory of what I topped out at when a gun was present during a game. I was very young for my class and technically was a class ahead of where I should have been, so the age may be more important than year in school :
11: 55 mph (probably cruised 52-53)
12: 60 mph (cruised 57-58)
13: 70 mph (cruised 67-68)
14: 75 mph (HS freshman) (cruised low 70s)
15: 80 (cruised around 78-79)
16: (HS Junior) 84, cruised 80-83
17: (HS Senior) 86, cruised 83-84. Hurt my arm 4 weeks into the season and did not pitch the rest of my high school career. I am guessing all things going well, would have topped off at 87 or 88, as I was just starting to get strong that season.
18: (College freshman): 86, cruised 83-85
19: (Soph) 88, cruised 85-86
20: (Junior) 90 (may have touched 91 or 92, but played in front of a LOT less scouts and our coach didn't believe in having us clocked).
My labrum went bye bye early April of junior year. I probably only gained 4 miles per hour between my high school senior year and college junior years. While some may attribute that to reaching a plateau, I think there would have been much more in the tank had I been healthy. After hurting my arm my senior year, got an MRI and my rotator cuff was severely inflamed. Being naive and not thinking long term, got a cortisone shot, got some celebrex, and went on my merry way to college. Never found out what the root of the issue was, and spent a lot of college pitching through pain or a dead arm. The ice, visits to the trainer and band work kept it manageable, and I never really divulged how bad it was unless it was unbearable, and then id just blame it on a "dead arm" spell.