This myth of 18 year olds being turned away to assess how much they want the job (i.e. see if they return) is nothing short of illegal. It doesn't mean they don't do it, mind you, but I can only hope and assume that they do not.
I think that the main reason youngsters (like myself) get turned away is completely down to lack of experience for both the interviews and "life experience".
From my 18th birthday to my 21st birthday, my life has absorbed so much from varying jobs and new and lost relationships.
If I was subjected to a Stage 2 at 18, I can't think of how I could answer half the questions with any sort of substance. Hell, I even failed an interview for cabin crew with MyTravel first time round from lack of experience. The interviewer would love to pass you, but they need to get out of you (at S2 at least) *proof* that you can cope with certain situations. They cannot pass you unless you prove to them you possess certain qualities, and I am not sure "Well, one time in English class..." would cover it.
Also, chances are at 18, NATS is your first big interview. It will come as a shock to you how tough it is...Second time round you will be a lot more prepared for the types of questions that will be thrown your way. You should have already thought out the answers you need to have rolling off the tip of your tounge to impress the interviewer.
On the same note, any 18 year old who can present themselves well will get through. Just prepare, prepare, prepare. Not only from PPRUNE, but search Google for "Common Job Interview Questions" etc. These will be similar to HR "interrigation" and even if they are not related it won't do you any harm preparing the answer anyway.
Also, try and vary your answers from question to question. If one answer is suitable for multiple questions (eg. Give example of time you were in charge/Give example of time you adapted to changing circumstances) then link them, and assess different qualities you possesesd for both situations. Then find another seperate answer so that you can use both answers and tell the examiner "I have another example if you would like it..."
You can do it, other people have. But it isn't an easy ride, not for anyone.. It just seems that once you've had a couple of interviews you begin to understand how to prepare better.
Good luck!
Andy