Is it possible malar festoons are due to sun exposure, or is it more likely that they are a genetic predisposition at 20 yo?

Festoons are often hereditary, but can be more noticeable with fluid retention from your diet.  To control them at this time, you can try to limit your sodium intake.  From a treatment standpoint, they are difficult.  You may find some improvement with lasers.  I recommend meeting with a board certified plastic surgeon for an evaluation and to formulate a customized treatment plan.

 At your age mailer bags are usually due to water retention. This is accentuated if you eat salty food the night before. Also sleeping on your face can accentuate these bags. The non-surgical solution to reduce these bags is to avoid salty food, drink plenty of water, sleep with your head elevated with pillow or pillows and definitely do not sleep on your face. Once the  fluid retention is eliminated, small amount of filters judiciously please can reduce the appearance of these bags. Otherwise surgical approach for tightening lower lid in an expert hands can also be a soloution

Malar festoons oftentimes become worse with increased salt and water retention.  They are not caused by sun exposure and as you have guessed, they are often hereditary.While there are no 100% good answers for them, I have seen some nice improvement with mid-face lift surgery, which will tighten the lower eyelid muscle that partially causes the festoons.  Best to consult with a plastic surgeon.
Elliot W. Jacobs, MD, FACS
New York City