To understand facial ageing, you have to look at the anatomy of the face. From the surface to the depth, we find the skin, then a layer of fibrous fat (the SMAS), then the smile muscles and finally the bones. The deep plane is located just below the SMAS, immediately above the mimetic muscles. It is devoid of blood vessels and is a natural gliding plane for the cheek.
Over the years, two major natural forces will cause the cheek to descend along the natural gliding plane of the deep plane: gravity and the thousands of contractions of the muscles of the mouth. The fat in the cheek will go down but it will be blocked by ligaments. It is because of this that the nasolabial fold tends to worsen over the years and that the bitterness fold, and the cheek appear.