Not sure if it is the same kid roothog is referring to, but I have seen a 12 year old touch 84 mph. This was very unusual and looked extremely dangerous.
Just because we have the luxury of seeing many of the best kids, I have seen many 14 year olds throwing 80, even seen a couple touching 90 or better at that age. One of those is now a senior in high school throwing in the upper 90s. Possible 1st round pick this coming June. If he were a bit taller, he would be cinch 1st round.
Each year at our National showcase we will have a very large number of kids just finishing their junior year throwing 90 or better. In fact, almost all of them throw 90.
However, if you took into account every pitcher of any age group, this large number of high velocity guys would represent a small percentage of all the pitchers. And some states might have a couple dozen while other states might not have one. Then the state that doesn't have one, might have 5 or 6 the next year.