It could be simply lack of appropriate exercise, though osteo is probably involved. Cycling only moves our joints through a small range of motion, only in one plane, and at rather low forces. That's not good in the long run. I suggest attacking on several fronts at once and not bothering with the scientific method here. You can worry about that later, if a combination of things works. I figure I don't have all that much time to fool around with stuff.
1) Stretching, to move your joints through ranges of motion they seldom experience: These stretches have worked for me:
https://www.bikeforums.net/road-cycl...l#post15372967
2) Training. Try doing deep squats, all the way down until your hams bottom out on your calves. If you feel discomfort on the way down, definitely don't go as far as pain. Stop at discomfort, and try to gradually increase squat depth over time. Try sets of 12, no weight. A good way to start out is to sit down far enough to just touch a chair, then back up.
3) Supplements. Many people respond to supplements, some don't. My wife and I happen to be responders. 1g glucosamine sulfate 2Xday, 1g MSM (methylsulfonylmethane) 1Xday. It takes at least a month of this before anything noticeable happens.