I’m 67, male, and I can still sit like that; just not as long as I used to.
If your just 64 and can’t sit like that, I would advise you get up and move a lot more.
If your a vet, try some of the old rifle range snapping in positions.
If you cant get into them, you need to do a lot more physical labor.
Just a suggestion.
For the first time in my life I just tried to sit like that. I was sort of able to, but my body leans to the left. You’re right that I need to get more physically active. Just a few months ago I was barely able to walk because of leg and back pain. Diagnosed with fibromyalgia I was put on Lyrica, which helped a lot. Right now I’m just happy to be able to climb stairs. Hoping to win back enough stamina to be able to run a stress test my cardiologist says I should have.