Good observation. I noticed in some of the autoimmune disease forums, that endurance athletes, especially aging ones, seemed over represented. Hmm. There are lots of average people and those that were sick their entire lives, but a significant proportion seem to have been steady exercisers.

I did sprint triathlons until age 50, and before I got into that, cycling, not minor mileage, either, usually 150 miles per week. I noticed after age 40 that something was wrong, but kept it up. Around age 47, something was undeniably wrong and others could tell. Now it is tough to move.

It may be the chronic inflammation that daily high intensity exercise causes.