Plenty. They don't go crazy every time they strike out or miss their pitch.
All you have to do is watch Bryce. Watch what happens when he gets flustered. It completely and utterly controls him. Sometimes for the entire game. Honestly, I think as soon as Davey sees this, he should just pull him.
19 year olds who can't control themselves don't magically become better at 22. It usually progresses.
You are lumping this to JUST 19 year old baseball players. I think you have some unrealistic expectations of late teenagers but that is just my own opinion. I'm 24 now and I realize what 19 meant to me. From my experience, myself and others at 19 were going off to college and getting away from home for the first time. No parents. No rules. I sure as hell thought I was still invincible from high school where nothing could hold me back and hadn't experienced the real world at all. I had friends that acted way worse than Bryce and have settled in quite nicely and mellowed out. Emotions are something we learn to deal with over time so I wouldn't be so pessimistic and short sighted on Harper.