Non-specific finger soreness

For the past 6 months or so, I've been experiencing mild finger joint stiffness and soreness for a day or two after a gym session. I'd like to understand it better to avoid making it worse, and possibly find treatments/training to mitigate it.

I'm 38. My gym workouts usually entail a few warmups on 10+/11, ~4 burns on onsights/projects (12- to 12+), and finish on a couple of steep 11's to failure. My projects are usually not on the most overhanging walls, and I'm not focusing on particularly crimpy or slopey routes, generally a broad mix of vertical to slightly overhanging routes. I always give myself at least one rest day between sessions.

I've been a lifelong climber (12+/13- at my peak), never had a finger injury, was out for a couple years for personal reasons, and have been base rebuilding for the past couple of years. I don't think it's a case of rapid gains or overtraining.

For a day or two after a session I'll have general finger stiffness and vague soreness in many/all of my fingers. No discernable swelling. With one rest day I'll regain ~80% strength, with 2 days it's %90, and 3 rest days I feel totally fresh again. All other joints feel fine.

Getting old? Arthritis? Diet? Under developed in some areas? I'd appreciate any suggestions, particularly from people that continue to climb strong in their 40's.