Arthritis can be managed.
I have VERY early osteoarthritis (15 years old) and I am taking glucosamine, which helps to make it quite manageable, also if you change your playing style so that movement of the fingers is as minimal as can be, you'll reduce the risk of running into troubles.
However, about 3 or 4 months ago, I was playing piano (my main instrument, sorry guys!) and was possibly into my 4th or 5th hour of practise for the day and my right index finger started hurting.... which was when i realised that I had been hammering that joint quite a lot...
THEN i did the dumb thing - i felt the pain go away a few hours later and i was straight back on the piano. Anyway, 3-4 months later (now) I have just gotten back from my physio who has told me to have a few months to let my joint heal
I may have done some permanent damage, which is awful because i am very very serious about music and i'm hoping to go to the conservatiorium after school...
so if something doesnt feel right:
step 1: STOP! and give it a few day's rest
step 2: Change the way you are playing