18 old and starting to get double chin. I have never been overweight. A few months ago I couldn't even get a double chin if I tucked my head back. I don't think it is due to overweight. However, I have had bad posture for years now. My head is always slouching over my shoulders. I don't know if this might have loosened my chin muscles and caused them to sag. On one picture my neck is relaxed on the other I flex my neck muscles (this is how it used to look relaxed).