I like to say that once you hit 30 you're old, then every 10 years thereafter you get another "really" in front of it. So at 40 you're really old, 50 it's really really old, 60 is really really old, etc. I find it amusing when people argue, to which my retort is that when any of us were ~20 we thought 30 was old, and that hasn't changed in the world...
I have a strange relationship with this thought. I grew up hanging around kids who were much older than me, and I looked a lot older than I was, so I didn't have that period a lot of kids do where they think 25 or 30 is old.
Also, my parents had me later than is typical (Mom was 35, Dad was 40), so they were in their 50s when I was a teen, and that just seemed like normal adult age to me.
If you had asked me at what age I considered someone to be old, I probably would have said around 60 for most of my life. I'm in my early 60s now, and I can't decide if I feel differently about it. I don't really have a problem considering myself old: my kids are adults, I'm getting ready to retire, etc. On the other hand, it feels like there's a huge difference between 60s old and 70s or 80s old.
Oh, and though my opinions on a lot of things have evolved over the decades, I don't feel different mentally (I don't just mean sharpness, but the way I approach things and my outlook). But my back is stiff if I sit for too long.
I was told, at 36, that I’m ‘still young’ by my doctor. Lady, I’m literally halfway through life if you assume the average American lifespan of 72. That’s not young. I’m LITERALLY middle-aged.
Oldness is relative, but the euphemisms for "old" are not. For example I'm pretty sure you don't get "advanced in years" until around the age of 60-65. But being "senior" feels more like 50 to me.