There was a significant amount of local talent concentrated in a 2-3 year age range when I was a kid where I lived. I don't think I ever saw a 12 year old throw 80, but at 13, I played with/against several in Pony league. Tom Wilhelmsen (Mariners) could do it fairly consistently. One of our teammates could as well (he quit ball as a HS frosh, probably throwing 84-85 when he did). My first best friend could, he was 90-94 his senior year and out of nowhere completely lost command of his fastball (ala Mark Wohlers) and fizzled out after one year at a D1 school in California. The hardest 12 year old I saw in person was probably 76-78. The legend on him was he could throw 80 at 12, but I never saw it. I did see him pitch in Pony All Stars as a 13 year old and I was sitting behind the gun getting him at 80 and 81 more than just once or twice. He didn't gain much more, he was still a really good pitcher in high school but was probably 87-89 and ended up being a JUCO shortstop.
I remember Ryan Schroyer (ASU, Red Sox AAA, HS teammate of Scott Hairston, Ian Kinsler, Chris & Shelley Duncan, Brian Anderson) throwing harder than all the guys I mentioned when he was 13, so I'd include him as well.
There were several others that were probably close to 80 at 13 but just a little shy (JJ Hardy, Brian Anderson (white sox), Jesus Cota (dbacks organization), Will Smith (Marlins/Texas) and a few others.