I was diagnosed with this when I was 13/14. I'm 22 next week and can't say it gives me much trouble anymore, don't think its been a problem since I was 18.
For most people, according to my Dr and Physio, it will improve if you engage fully with physio as it will strengthen muscles and improve alignment, and if you rest and take NSAID's if its painful. The problem for most people is that the muscles in the knee aren't strong enough so there is rubbing in your knee where contact shouldn't be made, and that's what causes the pain.
To the posters that haven't been to the GP, I would suggest you go as they can refer you to physio which makes the world of difference and it will really help improve things.