I did the whole recruitment process when I was 17, then backed out at the final stage. I just decided it wasn't for me. I done and passed all the tests, health checks, physicals etc. I suppose I just wasn't 100% committed to begin with, which you have to be.
It's a good choice though, and you can gain a lot of life experience and skills that you probably wouldn't get back home. You have obviously thought about it a lot, and are taking it seriously. Good Luck to you mate. I wish you all the best with it.
We will miss you around here if you go (well, I know I will mate!
).