I did mine a bit of damage when I was sixteen playing football, every now and then I'd get a twinge from it, but that seemed to go away once I started to go to the gym again.
I started running about six months ago and didn't have any problems until three months when I had a pain quite a lot of the time. It went on it's own after about two months, but if I try to run it comes back.
The doctor had a twist of it this morning and reckons it feels as it has arthritis starting.
The funniest thing was I sat down and told him it was my left knee, and he looked straight at my notes and said, "ah, you were diagnosed with arthritis in your right knee in 2002."
That was news to me as it's never given me a problem, and when he looked further my notes on it were blank. Hmm!