I only have anecdotes to support my post, however a colleague of mine recently joined a major European short haul at the age of 37. This was from an unrelated industry, modular route. Also have an ex colleague in the Integrated system who is in his mid 40s.
Im training now (PPL with a view to going commercial via modular) and everybody I speak to, be it instructors with years experience, current airline pilots, as well as club members are all very positive about the prospects. I’m 33 so likely to be 35/36 when I finish. If I have missed the boat in terms of the industry taking a downturn then I’ll keep my current job and become an instructor part time until it picks up again.
Be aware that the pilot training industry is just that. I’d recommend going to one of the Pilot Careers Live events with open eyes. Plenty of schools will happily take your money. Wings alliance is worth a look as well.
I ‘get’ that the industry has to bottom feed to provide people for the senior positions of the future, but from the research I have done, being under 40 won’t stop you on its own.