Most velocity gains I'm guessing is attributed to growth spurts. A 5'11 170lb kid is probably going to throw way harder than his former 5'2 120lb self. I'm curious if any adult has ever tried a velocity program without a previous injury that increased their velocity?
When asked what we did for our son my top answer was fed him and watched him grow. Once in high school he had hitting lessons to optimize his swing, pitching lessons to optimize his pitching mechanics and velocity and did a speed camp geared towards running the sixty.
Go figure he was a different player as a 6'1" 170 junior than a 5'11" 135 freshman (grew to 6'2" 195) I used to enjoy teasing him his sister was a 5'10" 140 pound freshman (played college softball at 150).