Is this pre-jowling? If so, what could the cause be in someone in their 20s? (Photo)

I find that fillers/fat in this area really help with lifting and contouring of the cheek and jawline in younger patients.  Implants and lifting can be done but I suggest a formal consultation.  Best, Dr. Emer.

 Following the beauty principles outlined in my book on face and
body beauty, male faces look ruggedly handsome when there is angularity in the
cheeks, chin and mandibular angles.  In addition to chin augmentation,
cheek augmentation with either a dermal filler or cheek implants (for a
permanent enhancement) will add angularity to the cheeks.  Mandibular
angle augmentation with a dermal filler (jaw implants IMHO have too many
unwanted side effects) will add volume, angularity and flare to the jaw line. From the photo, there are no "jowls" but what appears to be some residual fat below and in front of the cheeks.  I'd like to see a profile view to rule out a weak chin.  A weak chin creates an imbalance that makes the nose appear larger,
the lower face short and the mid face appear top heavy.  Chin augmentation,
using a chin implant, will add projection to the chin creating harmony and
balance to the lower face.  I have found placing a silastic chin implant
through a small, curved, incisions under the chin to be very safe, quick,
highly effective and far less invasive than a sliding genioplasty that requires
bone cuts with placement of screws and metal plates. In 20 year olds, pre-jowls and jowls indicate a lack of cheek bone volume and support anteriorly (in the front) allowing tissues to fall and sag off the cheeks towards the nose (creates nasolabial folds) and the mouth (creates the jowls). 

Hope this helps.