Yeah, I think roothog is spot on. My oldest son had his ups and downs emotionally around 13/14u timeframe. I think they loose brain cells as they progress through their teen years.
Aside from that, you also take into the fact that this is now "year two" on the 60/90 field. 13u baseball is the introduction of the 60/90 for most baseball players which is a big learning curve for all of them so the game is slowed down considerably. By 14u many of them have grown into the field, physically and mentally and the game pace picks up a bit. I would think that those that are frustrated at the 14u level are either A) late bloomers and could catch up in a few months once nature takes its course. B) Scared because they fear their playing days coming to an end because they don't stack up, too much stress (internal [self] and external [parents]) to perform.