Don't get me wrong as I don't work for NATS, but as far as I'm aware they are not ageist against 18 year olds at all.
If you pass all of their selection tests, then they will offer you a place. However as an 18 year old there is no choice but to be streamed as an Area Controller.
You may already know, NATS have 3 courses which they send people to complete at the college. Aerodrome, Aerodrome/Approach or Area.
If you follow the Area course, chances are you won't validate for longer than the other disciplines, which gives you time to hit the right age numbers.
As far as I am aware, being 18 is not a disadvantage to joining NATS.
Phil