Can my joints be harmed by playing guitar?

Probably not..: Playing the guitar has not been shown to be a significant risk for joint disease, unless you have tendency to develope arthritis anyway. In that case, the arthritis may come a bit earlier because of excessive use/movement of the joints that are pre-disposed to disease. If you have arthritic hands at age 39, consult doc for eval because i would not blame the guitar for it. Consult doc. Good luck.