The lens is a bi-concave structure suspended from the inner surface of the ciliary body of the eye. It functions to regulate the entry of light into the eye, and invert light signals for transmission to the light-sensing retina. The shape of the lens can undergo minor changes in response to the ciliary muscle, to allow for proper focusing on objects.
As an individual ages, the flexibility of the lens declines. By age 50, the lens typically cannot change shape. The risk of cataracts growing on the lens and impairing vision also increases with age.
For more information, see The Eye and Vision.