My son has had some sort of one on one hitting instructor since he was 11 or 12. When he was younger, I wouldn't say we made constant use of him, but we did make fairly regular use of him. And he definitely helped.
Eventually, we got hooked up with a guy many regard as the top hitting instructor in our area, a guy with many elite and highly successful clients from all over the area. The Instruction was at an entirely different level. He is definitely the kid's goto guy on all things hitting related. The kid tries to (politely and with a smile) tune out every other voice when it comes to hitting.
After the kid's freshman year, he also started seeing a strength and conditioning coach, (who was also a defensive skills specialist), He remains a bulk challenged dude, but this summer he is working his rear off in the weight room with his personal trainer and on his throwing program with his throwing partner.
Finally for awhile during his junior year and the summer after he was working with a sprint coach (the head coach of our hs track team) cause although the kid is very game fast, his 60 times did not seem to reflect his true abilities. Sprint coach taught him a lot and tried to recruit him for both his club track team and the high school track team. (Told the kid he had the potential to become an elite sprinter.)
In general, I think once they get truly serious about the game and are in a position to benefit from and listen to high level instruction, that's the time to do it.