Sunken cheeks occur from a loss of subcutaneous fat around the mouth and eyes, whether from weight loss, illness or aging. Years of smoking tobacco and exposure to harsh weather also diminishes delivery of nutrients from tiny blood vessels to the skin, causing it to lose elasticity.
Collagen and elastin are proteins that help make skin look plump and firm. As humans age, the skin produces less of these proteins. At the same time, the cheeks lose subcutaneous fat. In particular, between the ages of 30 and 40, the malar fat pads in the cheeks lose their fight with gravity and descend, enhancing nasolabial lines between the nose and lips and producing a trademark concavity.