Fungo - I'm still waiting for you to adopt mine so you can manage it all.
Seriously, very interesting answers so far. Taking a stab at it I'd say 12-15 would be one development stage then 16-18.
At 12-15 they're facing adjustments to bigger fields, rapidly changing bodies, stiffer and stiffer competition, maybe seeing regional or nationaly ranked teams instead of the locals, starting to see good fastballs, then different pitches (curve / changeup).
At 16-18 they're playing with the "big boys". They won't be playing with their peers in HS. They may be an underclassmen and get a chance to start. It will be a school sport instead of a travel team (part of the year anyway).
They'll only have about 30 games to accomplish their goals versus 50, 60, 70, 80 +.