28 year old, slim with weak jawline and double chin. (Photos)

I would suggest either a chin implant or a volume replacement filler such as Voluma to augment your weak chin.  Kybella is a simple office procedure which can reduce your double chin.

I suggest beginning with an in-person consultation to discuss your options. During your initial examination, your #chin will be evaluated along with the neck and jawline. A weak #chin implies a lack of chin support for the skin along the neck line. In younger individuals a chin implant alone is usually all that is needed to improve the appearance of the neck. Subsequently, it will also improve the overall #balance of the face and can create the illusion of a smaller nose. As a stand-alone procedure, chin augmentation can be performed in the office under local #anesthesia. For individuals with signs of #aging in the neck, a chin implant added to one of the recommended procedures above, will provide optimal results. Also, #liposuction under the chin is also another option your doctor can discuss. If you are not a candidate for chin or neck rejuvenation at the time of your #consultation, your board certified plastic surgeon can recommend alternatives to address your concern. 
However, there may be additional options to improve the appearance of your concern to improve the overall balance of your face. Such can be best evaluated in-person. Consulting with a board-certified plastic surgeon who specializes in #face, #neck and #chin procedures is important to ensure you are receiving the best advice, care, and surgical experience possible.

I find that VASER liposuction in combination with ThermiRF are best for contouring the chin in those who are candidates, but Kybella can be used as an alternative and needs to be combined with thermage/ultherapy/exilis or coolsculpting for good results. A chin implant would give more definition. I recommend getting a formal evaluation with a facial sculpting expert. Best, Dr. Emer