Skin ages in many identifiable ways.

External factors
Sun damage, pollution, free-radical damage, smoking.

Internal factors
Oxygen depletion, collagen destruction, reduced cell turnover, abnormal cell formation, decreased fat content, intercellular deficiency, hormone loss and depletion, chronological aging, immune suppression, and free radical damage.

Collagen and elastin break down
The support structures of the skin, collagen, and elastin break down and flatten as a result of repeated sun exposure. They also become less flexible and more hardened with age, so the skin becomes less elastic.

Fat cell depletion
Older skin looks more transparent and thinner than younger skin because younger skin has more fat cells in the dermis than older skin. It is also the reason overweight people tend to have fewer wrinkles. As we age, the skin keeps growing and expanding, even as the supporting fat tissues of the lower layers of skin are decreasing. Together with bone deterioration, fat cell depletion amidst continued skin production eventually causes the skin to sag. Simultaneously, the facial muscles lose their shape and firmness, giving the face a drooping appearance.

Intercellular structures
are reduced with age. The water-retaining and texture-enhancing elements in the intercellular structure such as ceramides, hyaluronic acids, polysaccharides, glycerin, and others are exhausted and not replenished. The skin’s support structures, collagen, and elastin deteriorate or are damaged. Older skin is also more subject to sensitivities and irritation than younger skin due to a weakening immune system and lowered surface defenses.

DNA and RNA shift with age.
On a deeper molecular level, the DNA and RNA genetic messages to the skin cell for reproduction slows down, and the cells stop reproducing as abundantly or as they did when we were younger. This change makes cells abnormally shaped, which further changes the texture of the skin and prevents the cells from retaining water. This is why older skin tends to be drier than younger skin. This change in the skin’s DNA and RNA happens as a result of sun damage and is believed to be a result of inflammatory response from free-radical damage built up in the skin cells over a while.