science

There's a huge difference between someone who hasn't taken care of themselves up to age 40 and someone who's hiking mountains at 75. I've seen some old ass 45 year olds and some young 80 year olds.

'And Just Like That' 's Leading Ladies Are the Same Age as the Characters in 'The Golden Girls'

Also, these guys are 35 and that hurts my feelings:

My perception is that I am old. (I am 39)

“This could have to do with the fact that many people do not want to be old, so they postpone the onset of old age,” said Wettstein

Pretty much this. I used to think that 50 was old. Now that I'm approaching it and know people past that age, I'm not sure I like that definition any more. I also don't "feel" old internally. Sure, my body isn't what it used to be. But, I am still active and haven't found myself limited in activities yet. Maybe that's coming. But, I'm also trying not to wreck the time I have left by being too stupid with my body. And I think that's where it's less about "being old" than it is being "used up". Sure, I did my share of stupid shit in my youth, we're all young and dumb at some point. With a bit of luck, we all get older. Hopefully, you take a few precautions and get lucky during that stupid stuff and you don't have a broken down body when you are older.

There is also a matter of experience and perspective. The more shit you live through, the more you are able to put life experiences in perspective. It's not only an age thing, being older usually means having lived through more things, but some folks get a lifetime of experience packed in a very short time due to bad circumstances. But, you start to recognize how little you can actually change or control in the world and start to accept the things you can't change. And maybe that's what "being old" is. You no longer have the vigor of youth nor the willingness to take on all the world's problems. You're more interested in just carving out a small patch of the world for yourself to live in as comfortably as you can. Sure, you may want a better world, and may even be roused to go do something about it from time to time. But you no longer believe that you can fix the world and really just want to warm your feet.

Gray hair = old. Simple as that. Don't like it, then stop being old!

You're as old as you feel! >Heel click<

~ a sarcastic leprechaun

I think Gogol (a classical Russian writer) called a 35 year old lady "an elderly woman". Never paid that close attention, but I imagine such occurrences can be found throughout classical literature.