There are several factors that cause your skin to wrinkle.

• Ageing: Collagen is a protein within your body that gives your skin
  structure. When your body’s cells age, their ability to produce collagen
  slows down, skin cells also divide more slowly, and the dermis begins to
  thin.

The dermis has a network of elastin and collagen fibres, which offer
support and elasticity. As this network loosens and unravels with time,
depressions form on your skin’s surface. Ageing skin is also less able
to retain moisture, less efficient in secreting oil and slower to heal,
contributing to the development of wrinkles.

• Facial muscle contractions: Small muscle contractions cause lines on
  your forehead, between your eyebrows (frown lines), and at the corners of
  your eyes (crow’s feet) to form. A person’s habitual facial expressions, such
  as smiling, frowning, squinting, and others, can make wrinkles in these
  areas more prominent.

• Sun damage: Ultraviolet (UV) radiation from the sun is cumulative and
  can prematurely age your skin—. Exposure to UV light breaks down your
  skin’s collagen and elastin fibres. Without supportive fibres, your skin
  loses strength and flexibility and begins to sag and wrinkle.

• Smoking: Harmful chemicals in tobacco accelerate the normal ageing
  process of your skin and damage the skin in a variety of ways. Researchers
  have found that nicotine in cigarettes causes narrowing of the blood
  vessels in the outermost layers of skin, this leads to a slower regeneration
  of new collagen and skin cells which results in wrinkles development.