My son is almost 6 and his knee pops when he straightens it. You can actually see the pop (if that makes sense). It jerks a little. He hasn t really complained about this until recently. Now he has started telling me that his knee hurts and he wants me to carry him, especially on days when he is really active. Do you have any ideas what this could be? He is very active - climbs trees and poles, bikes around, runs, etc.