Presbyopia: The loss of the eye's ability to change focus to see near objects. The reasons for this loss of the power of accommodation are not yet fully known. It is conventionally said to be due to the lens becoming less elastic with time.
The lens of the eye and the muscle that surrounds it are like atomic clocks, slowly ticking with such precision that a person's age can be determined by measuring their ability to focus close up.
Children can normally focus on objects an inch or so (a few centimeters) from their eyes. They lose that ability as teenagers. By age 45 or so, when the eyes can focus no closer than a yard (a meter) or two away, almost everyone needs glasses to read a newspaper or thread a needle.
The word "presbyopia" comes from the Greek for "elderly vision."