As we get older our skin produces less collagen, the protein that makes skin stretchy and elastic. Our skin also becomes thinner. These changes, together with movements of our muscles, facial expressions such as frowning or smiling and even gravity, cause the skin to wrinkle and sag.
Smoking, UV radiation in sunlight and not drinking enough water can cause wrinkles to develop earlier. Not sure if that is true for these wrinkles, though: