I have a dent in my chin which I didn't have in my teenage years. What caused it, and how can it be fixed? (Photo)

This is a normal anatomic line called the labiomental sulcus.  It is the point at which the base of the lower lip inside the mouth attaches to the jawbone and where the chin muscle attaches as well.  It frequently becomes deeper and more noticeable with age as our faces lose normal fat and the skin becomes thinner.  It is easy to reduce the depth and appearance of this line with injectable fillers such as Restylane which is the treatment I recommend, although future treatments are necessary.  Fat grafting works as well but is more involved, costlier, and a little less precise.