Sunken cheeks usually occur during the aging process. As we age, we tend to lose subcutaneous fat (fat that is just beneath the surface of the skin) from around our mouth and eye area. Because our bone structure never changes, the result of the loss of facial fat in those areas tends to give you sunken cheeks. Additionally, ageing reduces the amount of collagen and elastin generated by the skin which leads to thinner and less pliable skin that is more prone to wrinkling and sagging of the skin. Rapid weight loss and chronic illness are also other common causes of sunken cheeks.