Without the benefit of a picture, I can discuss several options to treat droopy corners of the mouth:(1) Injecting filler into the marionette lines to improve volume and lift the outer lip corners.(2) Injecting Botox into the Depressor Anguli Oris.(3) Surgery to removing the upper lip corner -- this may cause scarring.(4) Surgical incision of the DAO itself -- this muscle may still reattach and your corner drooping may recur.I commonly use a combination of the first two techniques -- Botox and filler. In your case, however, fillers may not help since, at your young age, you are unlikely to have significant skin folding there, and as you stated, repleting volume is not likely the issue. Botox might be your best option...Be sure to seek the services of an experienced physician injector. I think the key with Botox lies in truly understanding the anatomy of the injected area, and more importantly the variability in the anatomy between patients -- for brows, the forehead, and anywhere else you plan on receiving a Botox injection. This includes having a firm understanding of the origin, insertion, and action of each muscle that will be injected, the thickness of each muscle targeted, how deep beneath the skin the actual muscle resides, and the patient variability therein. As an aesthetic-trained plastic surgeon, I am intrinsically biased since I operate in the area for browlifts and facelifts, and have a unique perspective to the muscle anatomy since I commonly dissect under the skin, see the actual muscles themselves, and learn "first-hand" the incredible variability between patients -- live, "on the OR table" -- as opposed to via lectures or a cadaver dissection. For me, this helps guide where to inject and where not to. However, with that said, I know many non-aesthetic trained plastic surgeons and other physicians who know the anatomy well despite not operating in that area, and get good results.Good luck.Dr Markarian