How are wrinkles formed?

Aging
Wrinkles are a by-product of aging. As skin ages, the skin cells divide more slowly, and the inner layer (dermis) thins. The network of elastin and collagen fibers, which support the outer layer, loosens and unravels, resulting in depressions on the surface. Skin also loses its elasticity with age and is less able to retain moisture. Oil-secreting glands are less efficient and the skin is slower to heal.
 
Facial muscle contractions
Lines between the eyebrows (frown lines) and lines jutting from the corner of the eye (crows feet) tend to result from small muscle contractions. Habitual facial expressions like smiling, frowning, squinting make these wrinkles more prominent. Over time the expressions coupled with gravity contribute to  jowls and drooping eyelids.
 
Sun damage
Excessive exposure to ultraviolet radiation can lead to premature aging of skin, called photoaging. The ultraviolet sunrays that cause photoaging damage collagen fibers (the major structural proteins in the skin) and cause the excessive production of abnormal elastin (the protein which causes skin to stretch). When ultraviolet light damages skin tissue, an enzyme called metalloproteinases is produced. This enzyme creates and reforms collagen. During the process, however, some healthy collagen fibers are damaged, resulting in a disorganized formation of fibers called solar scars. Wrinkles develop when the rebuilding process occurs over and over.
 
Smoking
Healthy skin perpetually regenerates. While old collagen is broken down and removed, new collagen is being produced and installed. Smoke causes a marked reduction in new collagen production. A lack of new collagen results in the development of wrinkles.