My training came from my Dad hitting ground balls and fly balls to me and my brother when we were really little. He coached a fast pitch softball team, but I don't think that we had "finer points," just repetition. I guess that we had a little ability and hand/eye co-ordination, my brother more than me. I am sure that Dad's work with us helped us a bunch.LOL, For most these things come naturally. Other kids just have that big stick and never had to worry about mastering some of the finer points. You would be surprised at the 15yr olds I have seen.
I used to spend hours throwing a rubber ball or tennis ball against a 2 story brick building next door, and catching the rebounds, trying to make it as hard as possible. I would also throw up rocks and hit them with a broom handle. A baseball and a bat look pretty big after that. Such was practice in the 50's.