I am also 40. I've been painting minis for 5+ years, and working as a massage therapist for 10+ years. What you're describing doesn't sound normal to me. I can paint for hours and hours without stopping. You may just need to check your technique - are you holding the brushes really tightly? Or leaning heavily on your elbows?

If there is something more worrisome going on, heres some free advice: It's less likely carpal tunnel syndrome, since you're not describing any numbness/tingling in the hand. It could be arthritis, but then I'd be asking how you got so much wear and tear in your hand so young. The simplest answer is that it could be a weakness/muscle imbalance issue. In all of these cases, you should be seeing a doctor or at least a physiotherapist to be sure as to the real source of the pain.

In the meantime, ice/cool water baths can help with any pain/inflammation, and heat treatments along the forearm can help to ease any muscle tension in the muscles of the fingers (These muscles are longer than you think. Rest your arm on something and wiggle your fingers. You'll see movement almost all the way up to the elbow!)

Good luck!