I currently work for the company aged 18... although not an ATCO... I'm a trainee ATCE (Engineer).
From my time at CTC (where the College of Air Traffic Control now is) I found most trainee ATCO's to be in their mid twenties. From speaking to ATCO's I've found most have suggested gaining life experience and learning to cope with criticism which seems to be a valuable quality to hold. I'm not saying at 18 that you can't and you do get three attempts at the assessments but that's the best advice I can give to a fellow young person