24 years old and knees constantly hurt?

I'm only 24 years old, and it started out with only one knee hurting occassionally but has now moved on to both knees and hurting constantly. They hurt when I sit, they hurt when I walk, and they hurt most of all when I bend down or go up/down stairs. It hurts so much I can barely get up from a crouch. I'm not overweight and I don't do overly strenuous activities. I'm a web developer and sit most of my day. I used to cheerlead and was a base, where I would lift people up when I was in highschool, so I'm not sure if that is affecting me now, or if it is something entirely different. I heard from somewhere that I should work out my quads more, but will this damage my knees even more if they are hurting while I work out? Has anyone else had this problem, what is wrong with me?