I'm 20 and I appear to have sagging jowls. What can I do to improve my jawline? (photos)

Although there are surgical procedure available to modify the appearance of the mouth and jowls, none would I recommend to someone of your young age.  I think most of what you are visualizing is residual baby-fat and normal genetic variance.  Glad to help. 

For a more defined jawline to strengthen your profile (and help prevent the dreaded jawline "jowl" sagging), we use injectable fillers like Revanesse Versa - a filler known to limit swelling and give subtle, soft enhancements - or Radiesse. Using a cannula to limit pain and swelling, we inject on the bone in strategic points to get more lift and shaping, resulting in a more contoured lower face and defined, structured jawline. I would suggest getting a formal evaluation with a facial sculpting expert. Best, Dr. Emer

RF therapies and minor anterior facelifting options are available to you. But best to seek IN PERSON consultations... 

The jawline in your age group depends upon good definition from the 'angle' to the chin. If there is a lack of prominence/definition, the soft tissue/jowls/cheeks will change the contour. Review the area of the chin and the 'side of the chin' with a consideration for a genioplasty or parasymphyseal augmentation/side of chin.