Life expectancy is somewhere roughly in the 70s-to-80s. So it's not entirely surprising that if you tell someone "statistically, I have as much remaining time to live as you've been alive" they're going to think you're old.
Now if you actually step back and move the early part of life where you can't do much you can see it's much closer to middle-age than it is to death.
Interesting. I could tell that to myself, but I don't feel old.
In my world 50 isn't that old in this day and age. Most people I know at an age around 50 are healthy, active and enjoy life often even more then people in their 30s with young kids and everything else around that.
Professional (Fussell's Upper-Middle) class or higher? That's wildly different from the more Middle-and-under sorts of 50-year-olds I'm familiar with. That's the decade when they tend to start to look tired all the time (more so, I mean—that kind of "oh they're finally permanently broken" tired) and start having health issues & surgeries that seriously damage quality-of-life.