After being through the interview process myself. I dont believe NATS would turn you down purely because of your age. If they deem you mature enough, you have the necessary qualities and the right amount of drive and commitment you will be accepted.
I would personally say, and this has been echoed in another thread, that you might want to go to university, do the degree, and see where that takes you. Im a great advocate for university as I believe it teaches you some great life skills. The time and oppurtunity to have fun and socialise* are great and worth doing. NATS will still be here if you dont get the grades you are predicted or if the degree isn't to your tastes, It will still be here after your degree as well. Its always best to have something to fall back on, especially as the college pass rates seem to be dropping.
In saying all that, if its something you really want to do, and you feel you are ready, then go for it! Good luck!
*get very drunk, sleep with lots of women and eat lots of microwavable lasagne