I assume you're talking about the fact a knuckleballer doesnt generate as much stress on the arm and can pitch later in age?
Even tho thats true, a 39-40 year old is a 39-40 year old and they experiance pretty much the same soreness to the rest of the body as most other guys that age. They will get back pain and feel other nagging soreness just out of being old for professional sports. Even Hurl himself spoke on this topic a few weeks ago saying his age might not effect his arm but the guy is going to still feel the pains of any other 40 year old guy.