rbinaz is right on the money, in my experience. I have a 14 year old 8th grader who is a catcher but pitches too.
Mid 70's is above average; there is one guy I've seen in my state the last two years touching 80 at the age of 14. My son is a low 70's pitcher, and he's a pretty big strong kid.
A 14 year old at 80 or above is certainly not unheard of, but, like rbinaz says, it is exceptional.