I'm a 36 year old man with an increasingly frequent complaint..... Often, when my legs have been still for a while - namely driving long distances, or sitting somewhere I can't stretch my legs out (cinema, aircraft, etc...) - I get an ache in my knees that escalates to moderate pain if left. The ache can be alleviated by stretching my legs out or getting moving, as can the more painful feeling - just over a few minutes longer.
As a bit of historic info - I have had an issue with my left knee for many years (since being a teenager) - I often get a mild sort of cramp feeling that I have to repeatedly stretch my leg out and curl it up several times until something clicks behind my kneecap. Then it feels fine again. (I have a faint memory of a comment being made about the cartilage in my knee when I was about 11). This has never really bothered me enough to get medical help - I have always had a fairly active lifestyle - cycling and running a lot - and had a physical career, but it never stoppped me going about my day.
My worry now is that the pain I feel when sat still for a while (2 hours +) is a lot worse and in both knees.
I do plan to visit my GP to discuss this but I dont know how long it will be before I get time to go.