My son is now 13 5'6 145ish. before this season when he was still 12 at try outs in Feb, he was clocked at 63. No warm ups, just heres a ball throw it kid kind of thing. By the time the season started, losening up. He was easily hitting the high 60's. Could he hit 70? Yeah probably but chances are it would be wild. He has definitely learned that location and speed changes are the key to success in his leagues. Rarely will you see 2 of the same pitches from him. On his team he became their shut down closer. In 24 games he gave up 8 walks all season.
He is probably the best pitcher in the league, but not the fastest. There were 12 and 13 year olds throwing in the 70's, but they had poor control.
The one thing he needs to work on is lengthening his stride. If he got his stride to where it should be, he would probably be hitting low 70's fairly consistently. This fall he moves up to 60' distance so it should be interesting.
He only really throws his fastball, and a really good change up. Once in a great while he will use a cutter on a better hitter. He gets movement on it enough to throw a good hitter off. I don't let him throw a curve or anything with any wrist rotation yet.