For years I rode 23" frames when I was younger, then a bike shop who was more serious about road bikes fitted me to a 25 1/2" frame Raleigh. Since then and for most of my adult life I rode 63-64cm frames As I got older and started to have knee and back issues riding got more and more difficult unless the bike fit me just right.
For what ever reason, one day I took an old Nishiki International 23" frame and did a straight bar conversion with a long seat post and stem. The conversion got me riding more as I got older. I've not lost any overall height but somehow have lost inseam size. I'm still a fraction shy of 6ft 3in tall, but can no longer stand over my old 25.5" Raleigh Pro. Now I notice that even 23" seems like too much to stand over. I struggle to even get on the Raleigh these days and I even sold two vintage 23" frame mountain bikes I had which were just too tall for me now.
As I get older I have pain in my knees, ankles and feet, and lately my elbows and hands when I ride. I find that foam grips help the hands, and making sure I get maximum extension when pedaling. My knees and ankles don't hurt while riding, its when I step off the bike it hits me. I often can barely walk and over the next half hour or so my knees start to really throb. The knees have gotten worse but first bothered me in my 30's but at least then it would subside after a bit of walking but now I'm lucky to make it to somewhere to sit down. I carry a cane for when I have to get off the bike. I quit riding motorcycles for the same reason, I was having trouble coming to a stop and keeping the bike upright at rest. I'd put my foot down and the pain would light me up so bad my knee would buckle. First it was my right knee, now its both, and my ankles.
I picked up an old Peugeot mixte to flip a few months ago but have been riding it myself quite a bit, for some reason the knee issues aren't as bad on that bike, despite it being a good bit heavier and on the small size for me.