Knee pain comes to usually with 3 condition.
1. Osteo arthritis: here the
cartilage in your joint is getting damage with reduce space in your joint and increase inflammation.
2. Knee cap movement disorder: also called as chondro mallesia of
patella. It happens due to over use or change in knee cap movement which creates disturbamce in biomechanics of your knee and will pain when ever you fold and straight your knee. specially coming down from stairs use to be painful.
3. Ligament injury: knee joint have highest number of ligament amongst the other joint of body even if there is damage to any ligament also there will be pain in knee.
How ever in your case it can not be osteo arthritis since you are too young to have it. So it may be knee cap movement disorder or ligament damage due to some sudden twist of knee joint.
I suggest to visit ortho or physio and get your knee physically examine properly by them. Here based in physical test and diagnosis they will suggest for
physiotherapy exercises and treatment you can follow it and soon it will be all right.
You can also use hot and
cold pack on you painful knee to have some more relief.
Follow suggestion step by step and I am sure it will be really useful for you.
Take care.