I recently sold a story with a MC in his 50s
Somewhere on this forum a 50 year old person screamed "Fifty is not old!"
Sorry. But, the point I was going to make earlier when I first saw this topic list was easily illustrated there. Old age is nearly entirely subjective. When you are 6, teenagers look old. When you are 20, 40 looks old. When you are 30....
I guess, you might not want to think of characters as "old" people, that implies a stereotypical-ness that doesn't have a whole lot of realism. Old people are just people- they have the same emotions (although research shows that they are actually MORE tolerant and less emotional- the sort of nothing new under the sun phenomenon), they just have seen A LOT more of life. People themselves haven't changed much over the millenia. If you are ever in doubt, read some very old literature like Herodotus, or...whatever. The scenery and technology changes but the basics of motivation, etc don't and won't unless we start grafting extra animal or insect traits into our genomes.
So, basically, reading any fiction from any era will give you a good taste of how cultures perceive the role of the aged in their societies- but the basics remain the same. I think the better question is how will the aged be percieved in the culture o the future? There will certainly be a whole lot more "old people" in the future. I wonder what that means for society as a whole? Will be wiser, sicker, slower, more peaceful or?