How can I get a more defined jawline?

I am a female and am in very good shape (I'm 5'5" and weigh around 120 so losing weight wouldn't do much here) but my jawline is slightly sagging. It has nothing to do with age since I'm only sixteen, but I would like to have a sleeker more defined jawline like Audrey Hepburn. Are there exercises I can do to help this? Surgery is not an option.