Twenty/twenty vision is what normal a 20-year-old can see at 20 feet. The eyes of a healthy 20-year-old adjust quickly and easily to changes in brightness in the environment.  As eyes age, they lose their elasticity, reducing their ability to accommodate easily.  Adaptation from one light level to another takes longer and the range of sensitivity drastically diminishes the ability to see at low light levels.  A 60-year-old needs 10 times as much light as a normal 20-year-old to perform the same seeing task with equal speed and accuracy.  To a large extent, older eyes are more notably affected or even disabled by glare.

(tagline) – Because eyes have a difficult time adjusting to varying degrees of light as we age, older eyes must be protected from glare and also require more light to complete a given task.