> a 25 year old in 2020 considers 35 old

Tangential, but as someone in their mid 20s I wouldn't think of a 35yo as old at all. In-shape 35yos can be incredibly healthy. I'd probably put "getting old" at late 40s, and even later if you've got good knees/back.

I recently had the opportunity to visit my alma mater after many years away and realized the professors are now the age I thought they were almost two decades ago when I was their student. It was wild to realize just how skewed my sense of age had been! Not only did I apparently think people in their 50s were "old," but I actually thought they must have been in their 70s!

You might be surprised what you can do even without good knees and back. My knees currently feel like a balloon is inflated in them every time I stand up. I lean forward and brace myself against the railing when climbing my stairs. Nonetheless, I largely just read Hacker News in the morning when I'm waiting to take a crap before I can leave and I'm about to go run 8 miles, which I should be able to do in about an hour at a fairly easy pace. My weekly distance runs are creeping up past 20 miles and my tempo runs close to 6 minutes per mile. This is the best I've been since high school.

As for my back, well, Mondays are supposed to be cross-training days and I typically use an indoor rower for about an hour or so, but had to cut it short because my lower back seized up. I've had multiple lumbar interbody fusions and currently have ten screws, two rods, and two metal spacers in my spine. I still lift six days a week anyway, not exactly heavy and it probably never will be (certainly not if I continue focusing as much as I do on running), but you do whatever you can do.

The biggest differences getting old are it takes longer to warm up, you're more sore at the end of a session, recovery takes longer, and injuries do not heal as fast, typically lingering for months rather than days. But it doesn't mean you stop functioning. You just take your time and build up slowly. You have to learn to set goals in terms of years and decades. I worked out with resistance bands for six months before I tried to lift barbells. I walked for four years before I tried to run again. For a long time, I thought I would never be able to, but here I am.

If you saw me out on the street, I'm sure you could tell I'm in my 40s, but I don't think you'd see me as degraded or old. I've definitely got some crow's feet around my eyes and lines on my forehead, but still no gray hairs, no bald spots or receding, and the body of an underwear model. I'm not as strong as I was in my 20s when I focused on lifting and I'll probably never be as fast as I was earlier when I was a two-time state champion in high school cross-country and still ran pretty seriously in college, though non-competitively. You have to learn how to accept some level of inevitable decline, but nonetheless do as much as you can given the limitations you can't get past.

My dad had knee problems when he was older because of a career with a lot of manual labour - that means when you do get old you are less mobile which leads to other health problems...

I'm in my 50s myself now and while I have some other health problems my knees appear to be fine - not something I can say about people who were in the infantry or played sports professionally who are now all suffering quite badly.