I'm almost 40, and I've played the piano for 35 years. I still play almost daily. I've been playing the guitar off and on for about 30 years, although at a lower level of proficiency than the piano.
I started getting arthritis in my hands about 8 years ago, which has resulted in my thumb rotating inward, and ulnar deviation at the wrist, so that instead of my middle finger aligning with my forearm my thumb does. I've somewhat compensated for this at the piano, but I've lost thumb dexterity and often get cramps and pain in my forearms from the misalignment; the impact on my guitar-playing has been much worse, to the point that I almost can't play at all.
Has anyone else experiencing this had success with technique modifications or anything else to mitigate the problem?