My chiropractor told me he has a patient who did permanent damage to her wrists by knitting. But honestly, that guy is always popping off with crazy stuff.
Any repetitive motion you do wears your joints out. My aunt was a horse trainer for about 40 years, and now her thumbs and fingers can't do buckles very well anymore (almost every piece of horse equipment involves a buckle or three!) She is only in her early 60s and very active.
It is important to stretch and take breaks even if your hands don't hurt. But do as I say, not as I do! I can never put my needles down!