That's a really good question! I do think partly it's a matter of perspective.. I have been called OLD, and I jokingly say I am around 2 million years old... but to me, old is something like 80+ I am not sure why that number though... it's probably partly also to do with whether one acts old, whether they can walk or shuffle instead... whether they are active or sitting in a corner..
A small story... my son and I were discussing what we should do with me when I am old... I said, 'Oh, I don't care, just shove me in an old people's home when I am too old and I will sit happily in a corner where I can drool quietly.' He replied, 'But mum, you already drool.'
Completely depends on the person. I know 80-somethings who I don't think of as old at all. On the other hand, I know people 20 years younger than that that I do think of as old, because I think they "act" old.
If I had to say an age, I guess I'd go with 90. That puts me barely middle-aged and I guess I can live with that. :)
A man is old when he looks at a pretty girl and decides he's too tired. If he doesn't even look, push him over and bury him - he's failed to notice that he's dead.