What causes a pain in the knees?

Many causes: Osteo or rheumatoid arthritis, chondromalasia, medial or lateral meniscus tear, collateral ligament tear, intra-articulate hemorrhage from trauma or bleeding disorders, joint effusion, and osgood schlater syndrome.

Many causes: There could be many reasons for your knee pains ranging from traumatic or non-traumatic causes. Early arthritis/wear/tear likely in your case. Other common causes include sprains, strains, bursitis, tendinitis, ligament injury or meniscus tear. Rheumatologic diseases could also cause knee pains. See an orthopedic surgeon for clarity in diagnosis.