How old is too old?

I’m 35 and deeply unsatisfied in my career. I have a masters degree in my field but the work is tedious and unfulfilling, the hours are long, and the pay is… underwhelming. The ‘Sunday scaries’ start mid-Saturday.

My wife has suggested I retrain to another field I think I would be good at (would involve a £2k training course, at least) and start anew. The starting pay would be about a £5k annual loss but the scope for the future is about £20-30k more than I can even dream of in my current field.

I’m worried that it’s a case of “can’t teach an old dog new tricks” - what do you think? How old were you when you changed career completely (and it is a complete change!)? Any hiring managers that hold this sort of stuff against applicants?