What causes dark circles under your eyes if you are still young?

Nasal stuffiness. Any illness that causes nasal stuffiness can cause the dark circles under your eyes. Allergic rhinitis or hay fever is one of the most common. For this reason they are often called "allergic shiners".
Boggy veins. There is a venous plexus, or cluster of veins, under the eyelids. Anything that irritates the eyes or the mucous membranes in the lower eyelids can cause those veins to get boggy and accentuate them. Allergies of course can, but so can illness, and simple fatigue. Sleeplessness causes both eye strain and eye irritation, and rubbing can further irritation, causing more bogginess.