I'm only 20, however i've been experiencing stiff and painful joints, what could be the cause?

Mutiple causes: Joint inflammation(arthritis) is one cause of painful and stiff joints, but not the only cause. Without other clinical information it would be difficult to give a more specific answer. If these symptoms have been present for more than 6 to 8 weeks you should probably be evaluated by your physician.

Rheumatoid arthritis: It could be rheumatoid arthritis. Get checked out by a rheumatologist. They will usually obtain some blood work, that may detect a problem.