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.