As a person grows older, the lens grows larger and thicker and becomes far less elastic, partly because of progressive denaturation of the lens proteins.
The ability of the lens to change shape decreases with age. The power of accommodation decreases from about 14 diopters in a child to less than 2 diopters by the time a person reaches 45 to 50 years; it then decreases to essentially 0 diopters at age 70 years. Thereafter, the lens remains almost totally non accommodating, a condition known as “presbyopia.”
Once a person has reached the state of presbyopia, each eye remains focused permanently at an almost constant distance; this distance depends on the physical characteristics of each person’s eyes. The eyes can no longer accommodate for both near and far vision.
To see clearly both in the distance and nearby, an older person must wear bifocal glasses with the upper segment focused for far-seeing and the lower segment focused for near-seeing (e.g., for reading).