Youngest you can join is 13, youngest you can become enrolled is 13 and 3 months. The max age you can be in the Corps is 20 basically, it gets a little complicated though.
I wouldn't say 15/16 is too old, admittedly you may have a shorter time in the Corps but then again you have maturity on your side. I know guys that joined my sqn at 16 and are doing very well.
If I were you I would contact my nearest ATC unit soonest and talk to them, they will be able to set you straight. Don't hesitate as I am sure many people will agree with me in saying its one of the best things you can do between 13 and 20.
P.S. Just to give you a taster, I took these while flying with the ATC a while back:
http://www.cyborg2000.fsnet.co.uk
A.S.I