In the normal eye, light enters through the cornea (the clear front surface of the eye), passes through the crystalline lens inside the eye, and comes to a focus on the retina in the back of the eye. 

As we age, the lens inside the eye becomes less flexible. Presbyopia (Greek for "aging eyes") is an age-related condition in which the crystalline lens loses its elasticity and its ability to focus on near objects. This refractive error occurs naturally in men and women over the age of 40.

Presbyopia is usually corrected with reading glasses (whether over-the-counter or prescription) or contact lenses, but it can also be treated surgically.