Skin sagging or laxity is caused by loss of collagen and elastin in the skin, loss of and fat and even muscle.
This is caused by sun damage and age. The skin then becomes loose and pulls down with gravity.
Skin tightening procedures (Thermage, Refirme/Sublime, Apollo) were introduced a little over a decade ago and have been employed to reverse this for the last 10 years; before this surgery was the only option. Other things that can improve skin sagging include volumizing fillers (Restylane Lyft, Restylane, Sculptra) and laser resurfacing (CO2RE). Newer options can even improve deeper tissues leading to lifting as well as tightening.