Mid 40's is good for 10U around here. I've spent the last 3 years watching 10A and B ball and have only seen a couple in the low 50's. Those pitchers could throw it by almost anyone so they didn't develop any other pitches and are a lot less effective at the 12U level.
I cannot tell you how many 12U flame throwing phenoms I know that did not bother developing other pitches because they could just throw the ball by everyone. By 14U or 16U batters had caught up to the speed and they were rocked.
There are also a couple of flame throwers in college who never bothered to develop a change up because they have been blowing pitches by batters since they were 12. Guess what? SEC batters are trained to catch up to 70 MPH fastballs!