Why does my under eye/cheeks have such a big wrinkle/dent? (photo)

These are mild festoons or malar bags. At your age, fractional nonablative (clear and brilliant, Faxel) or radiofrequency (sublative, viva), co2 resurfacing and microneedling/prp will give great results without the need for surgery.  Fillers in this area help as well IF placed appropriately.  Please see someone with experience in treating this area. Best, Dr. Emer.

Dear Patient from NC, You seem to have what is referred to as a Malar Mound or festoon.These result from natural swelling above the deeper ligament in this region of the face.I've had success using lasers to tighten these from the outside.I'll include a video below. I hope this was helpful. 

You are correct that at age 19 your under eye bulge is not due to aging. It appears that you have an anatomical condition called a "tear trough deformity" and it is caused by a "negative vector orbiit".  Because the eyeball projects further than the underlying cheek bone, there is a hollow that can be corrected with the insertion of tear trough implants. Using undereye fillers will, in the long run, get very expensive because of the need to re-treat. I perform a large number of tear trough treatments including fat grafting and injectables, but it appears you may be an excellent candidate for implants. These are easily inserted through a lower lid blepharoplasty and the results are amazing.