I injured both of my knees when I was about 17 and had very early onset arthritis in both of them. What keeps them going is the right type of exercise for the condition (I am banned from anything involving steps) and keeping my weight under control. You'll probably need to do strengthening exercises the rest of your life if it is arthritis. It's easy to do and I know immediately if I start to slack.
Let us know how the physio goes!
Let us know how the physio goes!