37 years old and considering flight training in the US
People regret action in the short term and inaction in the long term.
If you don't give it a go, in 20 years you'll be more senior in your current field, richer, with a bigger house, moaning about the things that you still hate about your job now and wishing you'd given it a go.
If you do it, you'll probably be moaning about a whole new set of problems, but you'll be doing it on your terms, doing your dream job.
I am 30 and going to leave my corporate job this summer to pursue my dream. If it all goes to sh*t I will go back to my current job or find something else to do, but at least I'll have given it a go.
I suppose the one caveat here is that I don't have kids yet so don't have to worry about providing for them. If you have the means to do it assuming no income for 2 years with £40-80k expenditure, then £20-30k salary for 2 years then go for it.