Babies & young children have chubby cheeks. That is because a structure called the malar (cheek bone) fat pad rests firmly on top of the cheekbone in young children. Besides aging, there are many other reasons that attribute to hollow cheeks. The main underlying reason for it is the loss of skin elasticity & depletion of subcutaneous fat in facial skin. Skin ageing causes sagging of the facial skin. Gravity pulls the cheek skin down as we age and the cheek fat pad slides from the cheek into the face. The effect is a sunken cheek on & below the cheekbone where the fat pad used to be.