How man batters do you normally face in your first thee innings?
Why do I ask that?
Because in your first three innings, if you average out, 4 batters per inning, going into your fourth inning, you should be facing the middle and bottom of the order. Beyond that, the fifth in inning you should be facing the very bottom of the order, thus leaving your reliever to pick up the middle of the order and finish.
Also, for some reason getting out of an inning with only 10 pitches seems to be a bad thing - it’s not. If you can sustain yourself with 10 pitches per inning, I would say that you’re doing pretty darn good. So for five innings at your age, letting someone else take it from there and leaving the field with only 50 pitches, is nothing to sneeze at.
I should mention that, your level of competition and those who you are facing, is kind of difficult to judge using this medium. If you’re in a highly contested league, and you have a decent bullpen to draw on, with a reasonable skipper (head coach) then you should focus on what Jerp33 recommended. Just pace yourself and understand your limits, how healthy you feel going into a schedule and stay strictly to a days-rest regiment.