Human eyesight deteriorates with age because the essential parts of the eye lose freshness and functionality in time and with extended use. Examples of issues related to decreased function of a specific part of the eye include problems with near-sight vision (otherwise known as presbyopia) and issues reading small text, which is the result of a loss of flexibility in the eye's lens. Age-related diseases can also cause damage to the retina and optic nerve, although age alone does not necessarily damage these parts of the eye.
There are four main parts of the eye that contribute to the human ability to see, including the cornea (where light enters the eye), the lens (focuses the light entering the eye), the retina (converts light into a signal for the brain) and the optic nerve (carries the signal from the retina to the brain). In some cases, simple damage to the eye's anatomy cause issues with vision. For example, the cornea can become scratched with time, causing vision clarity issues much like the lens of an older camera that has seen heavy use and has become scratched, which causes distortions in photographs. However, age-related problems with eyesight can also be caused by other age-related physical ailments. For example, individuals with circulation problems or diabetes may experience vision problems caused by damage to the eye's nerves and blood vessels.