Skin ageing of facial skin involves ageing of multiple skin layers: the epidermis shows irregularities, the dermis become thinner and less elastic, the subcutaneous fat volume decreases and the fat pads slide downwards. But ageing also involves bone loss in facial bones leading to loss of support. This leads to excess skin and soft tissue accumulation near the jaw and chin leading to the formation of jowls. This is accentuated by the downward pull of the superficial neck muscles which also leads to laxity of the skin of the neck.
Saggy skin is less firm and less elastic skin. This is usually more pronounced in the lower face and neck and often make us look older and tired.