I think technically there's no upper age limit (in the UK at least) due to age discrimination laws. As far as I understand things, some (many?) ATCOs retire when they are in their late 50s (but I could be wrong about this), and you also need to factor in that it could be 2-3 years before you are validated. So, I'd say work out how many years that leaves you in between and decide if you think it's worth the investment in terms of time and money (because during the training years you have to get by on a very low salary). I'm in my late 30s... I'm not sure if that put me at the "older end of the age range" or not, as I know some trainees are older than me, but at the same time I'd never have been accepted by many ANSPs, such as Eurocontrol.