I've had it for close to 15 years - am nearly 50. Blood test - only 80% shows up in blood test. Depends if you have osteo or rheumatoid type (in general terms). See a rheumatologist. NSAIDS used to help but long term use put me on the road to gastritis - so beware of them. I now take Methotrexate.

As for playing - warming the fingers/wrists/bodies helps. I take a couple of paracetamol (acetaminophen in N/America), and use red tiger balm about 30 - 60 mins before I play. That seems to loosen the hand etc up.

Find alternate chords and/or change key (capo). I play more bass now - less stress on the hand. lighter gauge strings then lower the action. I'm teaching myself alt tunings and will likely have to take up slide. I used to fingerpick a lot. Consider a shorter scale guitar (don't have to stretch). As to the width of the fingerboard, I need a narrow one to play lead and a wider one to play chords (classical guitar width).

It's very much about finding what suits, looking after the joints, and getting GOOD medical help early before the deterioration gets too advanced. And as MikeL2 did, a friend of mine swapped to piano. Better some music than none!!

I did a lot of carpentry today and then tried to play guitar tonight - not a hope!