My 16 year old granddaughter has bags under her eyes, what could cause this?

1 of the most common possibilities that can cause this discoloration under the eyes called "allergic shiners". These are seen in patients have significant allergic rhinitis and sometimes accompanied by asthma as well. You might check with your pediatrician regarding this possibility.

Unless there is redness, itching or inflammation also present, it is unlikely to be something to be concerned about. If it's been present for a long time, it may be simply the way she looks. Are there pictures of other family members to compare with?