What causes skin to wrinkle with age?

​​What causes skin to wrinkle with age?

The skin is made up of three layers:
The outermost layer which everyone can see, called the Epidermis.
The middle layer, called the Dermis.
The innermost layer, called the Subcutaneous layer.

When we're young, we don't have wrinkles because the Skin does a great job of stretching and holding in moisture. The dermis has an elastic quality thanks to fibers called Elastin that keep the skin looking and feeling young. A protein in the dermis called Collagen also plays a part in preventing wrinkles.
But over time, the dermis loses both collagen and elastin, so skin gets thinner and has trouble getting enough moisture to the epidermis. The fat in the subcutaneous layer that gives skin a plump appearance also begins to disappear, the epidermis starts to Sag, and wrinkles form.