Even some young eyes can strain to see the small text of fiction and school text books but the majority of people start to need reading glasses as they pass the age of 40. Often the distance vision remains exceptional while the eyes struggle to correct for objects held in the hand.

Reading glasses are usually required because of "presbyopia" – "a gradual thickening and loss of flexibility of the natural lens inside your eye. These age-related changes occur within the proteins in the lens, making the lens harder and less elastic over time. Age-related changes also take place in the muscle fibers surrounding the lens. With less elasticity, the eye has a harder time focusing up close." Source: All About Vision - Presbyopia.

See also: The American Optometric Association – Presbyopia