Guess I inadvertently made an assumption about what you meant by “flying open”. To me, “flying open” refers to the glove arm swinging out to the side. That will often lead to early shoulder rotation which, it sounds, is what you were referring to. Early shoulder rotation often results in those high pitches or the “up & in” pitches.
The “stay back” terminology is often ambiguous. Some coaches/instructors use it to mean keeping the upper half back while the front hip gets out front while others use it to mean keeping the total body back as in the old balance point drill.
Growth spurts can certainly affect velocity and control until strength and flexibility catches up with the growth.