I've had 4 friends apply.
2 got in - both older than 21 (1 was 28 and the other 25)\
2 didn't - 1 was 19 and the other 22....
I don't know if their is a specific policy not to hire under 21's, however it might be worthwhile just waiting the few months until 21 rather than take the risk that they knock you out on age alone!
Good luck with it....