Every once in a while people argue about what it means to be "old". Some of my kids think that 50 is the cutoff. The term is relative and subjective, but it could be useful to think about it in more absolute terms.
Sometimes a teenager will look at a 35-year-old and say "ew, you're old". But what they really mean is "you're in a place in your life that I can't relate to." It says more about the younger person than the older one. So that's not a good place to look for a definition.
There are a few conditions I've observed that I think lend themselves to defining "oldness" in a few different ways.
The first sense is "physically old". We could define that as the point at which you can't physically do the stuff you used to, because your body's capabilities are shutting down. In that sense, most 35-year-olds aren't old, and neither are most 50-year-olds.
A similar standard can apply to "mentally old". That should mean that your brain doesn't work as well, to the point that your memory or cognitive abilities are an order of magnitude lower than they used to be.
Here's a more interesting one: I've observed that some people get to a point in their lives where they've made mistakes, and they continue that pattern of behavior even when it's clear that they'd be better off changing - but they can't let themselves change, because doing so would be to admit that all of the damage that has already been done was preventable. So they refuse to change, as a coping mechanism against the pain of admitting past mistakes. (This is different from simply refusing to admit you're wrong, which of course is present in everybody from time to time, regardless of age.) In my view, when you reach a point where your past mistakes add up to regret that you can't face, you're emotionally old.
There's another kind of oldness that I don't have a good name for. It's when there is no one in your life that you'd take advice from. It's not that you can't face your mistakes, it's just that you don't see anyone as an authority figure, or as having wisdom that you don't already have. I kind of think of this as being "spiritually old", since essentially spirituality is all about being willing to make changes even when you'd rather not.
You could also label the above state as being "intellectually old", although I think that might be better applied to another very common situation: feeling like you already know enough. That's closely connected to not seeing authority, but even someone who won't take advice can be interested in learning. And someone who always takes advice from someone might use that as an excuse from having to learn on their own. I'm not 100% sure that "oldness" is the right way to describe this state, since many people never really value learning, but everyone has phases of their lives where they have to learn something, even if it's just how to do a job. Once they stop feeling that need, I think they've lost something that's essentially youthful - the ability to look at the world as something you can explore.
I guess the point of all of this is that, while everybody will outlive their physical health, most of the other aspects can be avoided. (The mental thing is on the fence since it has a physiological causes, but some of that can be prevented too.) In theory, if you stay humble enough to learn and change, you never have to really feel old.
Not that I'd know - I'm still young.
