I believe Tai ji is very beneficial for health. But I dont think you have to actually study it to receive health benefits of chi. I dont study it personally I study mantis. But I can practice my set slow and work with the big circles. Deep movements. Coordinate the air with the movements and feel the chi pulse from dan tien out to the finger tips. Relaxed and aware. Stomach out for health and in for fighting. Im not putting tai ji down at all. This is more aimed torwards guys who dont practice tai ji and do gung fu. You can practice your gung fu and air soft to.
Now about arthritis. Yes soft moves help aleviate pain and increase mobility. But I would also look into Dit da jow, Tui na and Medicine. Never use ice. Cool jow that helps with inflamation and removes excess water from the joint. I believe its the excess fluid that makes joints hurt more in the cold. From what I have gathered most long term(not injury) phsyical problem stem from excess. Eating too much, hitting hard surfaces too much. etc... With out balance you become one sided and problems start to occur. Just my 2 cents. Peace