One of those age-old questions, without a clear answer: "Do you quit riding a bicycle because you get old, or do you get old because you quit riding a bicycle?"
I like to think it's the latter... and I hope to keep riding until my kids insist I give it up because I can no longer keep it upright! (And hoping I never arrive at that point...)
The mother-in-law lives with us... well, actually, adjacent to us. She's in her 90s, and her kids prevailed on her to sell her house and move in with one of them [my wife] where she could get attention and assistance when needed. We built a little 600-square-foot cottage for her out back, connected to the main house, and it's been mostly a comfortable transition.
However, I've observed her. When she first arrived in Boise a couple years ago, she would routinely cross the street and walk around the perimeter of the city park - about 1/2 mile. She wasn't setting any records for speed, but she was doing it. In the ensuing couple of years, she's declined noticeably. She mostly sits in her easy chair now. When she walks any distance, its with the assistance of her walker. (She still goes and "exercises" in a swimming pool a couple times a week, but that's about the extent of it.) She complains about her constantly-sore knees.
So - did she quit walking because she has sore knees, or does she have sore knees because she quit walking? She had more incentive to get up and active in the morning, before she took up housekeeping with us. Nowadays, if she chooses, she can mostly putter around in her place and her needs will be seen to by her house mates. There might be a lesson here. Or maybe not.
I'm 61. Which means I'll be middle-aged within ten years or so. (Hahahaha!) These days, when I drag my sorry carcass out of bed, it's a rare day when I don't feel somewhat sore and stiff. I notice it while doing the most trivial of things... making the bed, brushing my teeth. But after 10 or 15 minutes of working the kinks out, and an aspirin tablet or two now and then, I generally feel all right with the world. But I can see how one could take a different route: "I'm pretty stiff and sore today. I better take it easy and watch TV, and I'll probably feel better in the morning." But will you ever feel better in the morning, if your preferred remedy is to relax? I'm thinking not. "Use it or lose it." I see evidence of that every day... and I hope I can keep using it, because I don't want to lose it.