From the moment we are born, all our cells are going through a life cycle of their own and after 25 years of age, certain parts of the body start aging more rapidly. The skin is among them. The factors that control skin aging may lie within the body (intrinsic) or outside it (extrinsic).
Intrinsic or internal factors can be beyond our control. These include the usual wear and tear of cells, and inherent quality of some skin types. For example, darker skin / thicker skin ages slower than thinner paler skin types. Dry skin ages faster than oily skin.
The factors we can control are diet, sun exposure and sun damage, exercise and oxidative stress from lack of sleep / inadequate rest or smoking. Thus although aging is inevitable, the rate of aging can be controlled to some extent. For example, higher sugar intake causes faster aging. Smoking and lack of sleep cause oxidative stress and faster aging. Regular exercise helps slow down aging.
Treatments which boost the collagen remodelling process or boost hydration help by slowing down the rate of aging and undoing some of the cumulative damage from internal/external factors. Thus aging is dependant not only on our environment but on our interaction with the world too.