As we grow older, the eye's lens becomes less elastic. That means it loses its ability to change shape. The medical term for this is presbyopia. The condition is more noticeable when we try to see things close up, because the ciliary body must contract in order to make the lens thicker. The loss of elasticity keeps the lens from becoming thicker. When this happens, we lose the ability to focus on close objects.
Typically, when this first happens, people begin to hold things farther away while trying to see them in focus. By our mid-40s this usually has become quite noticeable. Eventually, the lens is unable to move. It becomes more or less permanently focused at one distance (that distance is different for each person).
Bifocals are required to help correct this. Bifocals are a combination of a lower lens to aid in close vision (such as reading) and an upper lens for distance vision.