Fullness under the chin is normally associated with three factors: aging, genetics and/or changes in weight.
As people get older, changes in the body often cause or increase a double chin. The reduction of muscle tone and thinning skin can contribute to sagging, especially underneath the chin. And for some people, the appearance of jowls can contribute to the appearance of a double chin as well.
Genetics can play an important role when it comes to having a double chin. Many younger patients can inherit a predisposition to fullness underneath the chin, and this can begin to show at a young age. Often, aging can augment these features.
Changes in weight can also be a common factor for people who have extra fullness under the chin, or a double chin. As people age, they often gain weight, and even a moderate weight gain may result in more noticeable fullness in the area under the chin.