Thanks for mentioning the updated documents - I have my S1 on Monday so that will be useful!
NATS must have to take into account that if you are only 18 then you're not going to have done as much as an older person who would have had a chance to get a good job and work their way up to a senior level.
I've tried to do as much as I can in a short space of time - I started my PPL when I was 16 and obtained my PPL when I was 17 and have a share in a C150 but I only go to college and work in a shop (to pay for flying). I hope that will help get me through the interview, that is assuming I pass S1.