I should hope not that they fail people 1st time around who are 18!
Well if they were to do so that is age discrimination which is obviously against the law, and something I expect NATS are particularly careful not to do considering their recent battle in court in which they had to remove the upper age limit.
And anyway, just do your best!
Are NATS good at spotting enthusiastic candidates and does this have a high postive effect on getting through each stage?
Surely if you're enthusiastic you're more likely to succeed at each stage anyway? I think its more motivation rather than enthusiasm that is one of the key qualities and attributes being saught. Make sure you show your enthusiasm throughout the process, bear in mind that won't get you through alone.