From the day they are born, every person has a certain amount of tension in their muscles, known as muscle tone, which is also present when the body is at rest. The natural movement of the muscles, for example when a person takes on a pensive expression, causes them to move to a greater or lesser degree, briefly forming wrinkles.
The skin is capable of compensating for this pattern of tension and release for a very long time. However, as skin elasticity decreases with age, there comes a point at which these expression-related lines no longer disappear. They become deeper and are visible as wrinkles in the skin even when the facial muscles are relaxed.