Gun readings at 9 or 10 years old don't really mean anything. Then again it would be nice information to have. I don't think much of that type data exists. I know I've never used a radar gun for kids that age.
The thing that would be most interesting is those that have become MLB pitchers. If they were pitching at 9 or 10, it would be interesting to know what their velocity was. Who knows, maybe that information could somehow prove valuable. Of course, the problem is, I'm guessing here, that not very many, if any, of those MLB pitchers were ever throwing to a gun when they were that age. But a good arm is a good arm, some just improve a lot more than others.
We have seen low 80s turn into mid 90s between sophomore and senior year. My guess is that normally the hardest throwing kid at 9 years old is far from the hardest thrower at 18 years old. But I don't know that for absolutely sure!