Inflammation. Knee pain is a young patient is most commonly caused by inflammation from bursitis, tendonitis, or cartilage degeneration. A good physical exam and possible imaging could be used to confirm the diagnosis. Less common causes of knee pain are irritated nerve roots from the lumbar spine, infection, or tumors.
Exam. There are numerous reasons for knee pain and an evaluation is generally required. Most orthopedics exams are directed at identifying surgical problems. Patients are often surprised much of knee pain is poor control at the hip and ankle. Two YouTube videos review the concepts: https://www.youtube.com/watch?v=43Nh5aYwFnA https://www.youtube.com/watch?v=t6SXiDCMP3E.
Knee pain. What about it :).
See your doctor. This is a problem that requires a face-to-face meeting with your doctor. In that meeting, your doctor will listen to you, perform a throrough examination and possibly order labs or other tests. Based on this information, he/she will be able to tell you what's wrong and what to do about it.