Lets face it, you can learn to fly at any age but the sad truth is that you pick things up a lot faster when you are young.
appart from the obvious physiological differences I would be interested to see how a prospective fast jet trainee who is 55 years old would go. not to well i would imagine.
The other isssue is that I believe that captaincy, SA and airmanship are all more easlily developed when you are 18. yes, there is the old 'but they are not as mature line'. it didn't seem to stop 18 year old spitfire pilots!
Anyway. There is no reason that a reasonable progression can be achieved at a more mature age.