How old is too old?

[Scooby Don't]

I started my course at 24 and was about the 4th oldest out of 30 or so starters. There were somewhat fewer finishers!

I really don't think the "return on training investment" thing is an issue. You are free to change employers, run away to the circus, become a househusband or whatever at any point after or during training. NATS may not like it, but I personally think that after 5 years of valid controlling, you've given a perfectly decent return of service. And that was on the basis of getting a good pension scheme and being treated with respect by management, both of which are questionable in NATS these days.

It's a fair point that younger brains are conditioned for learning, but it does depend on the individual and there is more to it than age-related learning ability. I worked harder at 24 than I would have at 18, and I'm pretty sure I would have worked harder still at 30! At 40, not that I'm there yet, I would not like to be starting out in ATC, but I don't see any problems with a motivated 30-year old candidate. Good luck!