A very interesting thread, thanks to the op who started it.
I must say that for me, out of all the replies and points put forward, Whirlybird's posts make the most interesting and indeed, sensible reading.
He/she is correct in the assumption that age affects everybody differently - indeed, I know 50 year olds who are old before their time (and 80 year olds who jump over their heads). Is it nature or nuture - the 'luck of the draw', genetic involvement. Who knows...?
The primary point I really agree with however, is there is a very big difference with those who fly for pleasure - even those who fly commerially (there are normally 2 of the on the flightdeck), than those who instruct for a living.
Whirlybirds comments strike a very valid chord there.
I would go on and elaborate my points, but am typing this on a very unresponsive android tablet, which is far too hard and taking far too long...