Why do your knees hurt when I walk up or down stairs?

i forgot to say that I have a bad knee from an injury from a couple of years ago. I am only 15 years old and I already have limitations for exercising. Yeah so sometimes in gym when we do a lot of stairs or run a lot or when I just exercise on my own, my knee starts to hurt. In gym when we run the bleachers, I dont do it. Today we were just walking them so I decided to walk them. Now my knee is bothering me.

I do suggest you see a doctor...