I'm a 21 female with knee pain. I've had knee pain off and on since I was 11. It used to come about in the Spring when I played sports. Now 21, it's much more consistent. Walking/exercise (at anything more than a casual pace) can trigger the onset of pain. Also standing for more than an hour (at work, for example) bothers my knee. It's generally not a sharp pain, more of a soreness or dull pain around the entire knee. I also can't sleep on my back without a pillow under my knee, the full extension bothers it. Once my knee starts bothering me, it snowballs to the point where I limp around.
I went to the doctor once when I was 15, but she did not take me seriously, because 1) I was a teenage girl with knee pain and 2) because it didn't hurt at the time. She wrote it off as growing pains, but it's been a while since I stopped growing...I'm young and fit, not over weight, so why does fulling extending my knee cause pain?