That crease between your lower lip and your chin? That is the mentolabial fold.
What is the mentolabial fold?
Like mentioned in the intro, the mentolabial fold is just the crease between the lower lip and the chin pad. It is also called the labiomental crease.
It is a natural feature, but in some cases it can be the following:
Why does it deepen?
That fold can deepen because of multiple factors. A recessed chin (lack of skeletal support) causes that sunken-in look.
Apart from that an overactive mentalis muscle can cause a look like that though only really insignificantly, you are better off focusing on the skeletal issues.
Soft tissue loss with age is another factor, it's not a correlation isn't causation type of thing but rather directly caused by aging in most cases. That's why there are many old people with deep mentolabial folds.
Mentolabial angle
The ideal mentolabial angle is around 110-130°.
If the angle is too sharp -> chin looks curled inward
If the angle is too flat -> no definition
Chin implants make it worse
Unironically a chin implant looks shit in most cases. Chin implants push the chin pad forward, not the base. Result? Your mentolabial fold becomes deeper because the soft tissue stretches and the fold gets exaggerated.
Never use implants on a recessed mandible, a genioplasty simply mogs.