As you age, the tissue structures and muscles supporting your eyelids weaken. The skin may start to sag, and fat that is normally confined to the area around the eye (orbit) can move into the area below your eyes. Also, the space below your eyes can accumulate fluid, making the under-eye area appear puffy or swollen.
Dark circles have a variety of different causes. Besides alcohol and lack of sleep, illnesses also cause these circles to appear. The combination of a fair complexion and thin skin if often the culprit.