TCR: There are definitely trainee's who have been accepted into the college at age 18, however the initial selection tests you have done will not 'filter' you out because of your age.
Even at interview stage you won't get rejected because of your age, unless of course NATS have good reason to so in your case, there's no way of telling. Keep at it though if you think you've got what it takes!
I've got stage 3 tomorrow, currently debating whether to go up to London tonight or on the actual day. FGW trains are running a normal service at the moment, so may end up coming up tomorrow...