My advice would be if Airlines is the dream, you should see if it realistic to have a job by the time you are 45. Expect 2 - 3 years from you finish training until you might get the chance of a job (this is in Euro land)
Starting training anytime in your 30's is not to late, even if you are 39, as long as you can see a realistic chance of a job at approx 45.
Any later I would not bother, the risk and time is to big. Cost vs earning is just not worth it, unless you have loads of spare cash and no family to worry about.