Exposure to ultraviolet light, UVA or UVB, from sunlight accounts for 90% of the symptoms of premature skin aging. Most of the photoaging effects occur by age 20. The amount of damage to the skin caused by the sun is determined by the total lifetime amount of radiation exposure and the person's pigment protection.

Changes in the epidermis caused by the sun include thinning of the epidermis and the growth of skin lesions such as actinic keratoses, basal cell carcinomas, and squamous cell carcinomas.

In the dermis, sun effects cause collagen to break down at a higher rate than with just chronologic aging. Sunlight damages collagen fibers and causes the accumulation of abnormal elastin. When this sun-induced elastin accumulates, enzymes called metalloproteinases are produced in large quantities. Normally, metalloproteinases remodel sun-injured skin by manufacturing and reforming collagen. However, this process does not always work well and some of the metalloproteinases actually break down collagen. This results in the formation of disorganized collagen fibers known as solar scars. When the skin repeats this imperfect rebuilding process over and over wrinkles develop.

Free Radicals and Wrinkles
Free radicals are unstable oxygen molecules that have only one electron instead of two. Because electrons are found in pairs the molecule must scavenge other molecules for another electron. When the second molecule looses its electron to the first molecule, it must then find another electron repeating the process. This process can damage cell function and alter genetic material. Free radical damage causes wrinkles by activating the metalloproteinases that break down collagen. There are several factors that start this cascading process including exposure to even small amounts of UV radiation in sunlight, smoking, and exposure to air pollution.

Hormone Effects and Wrinkles
It is likely that there are skin changes as a result of the hormonal effects of menopause or decreased estrogen production. However, studies in humans have not documented which skin changes are specific to decreased estrogen and which skin changes are a result of sun exposure or just normal chronological aging. In animal experiments lack of estrogen can cause a decrease in collagen levels of 2% per year and a decrease in skin thickness of 1% per year.

Muscle Use and Wrinkles
Habitual facial expressions cause the skin to wrinkle as it looses elasticity. Frown lines between the eyebrows and crows feet radiating from the corners of the eyes develop as the tiny muscles in those areas permanently contract.

Gravity and Wrinkles
The effects of gravity make the loosening of the skin more apparent as skin sags more. This causes jowls and drooping eyelids.