In youth, the natural (crystalline) lens of the eye is flexible and allows the eye to focus at near and far distances, a process called accommodation. This process involves the ciliary muscle inside the eye, which causes the lens to change shape and focus. Throughout life, the lens continues to add cells. Eventually, the lens becomes so rigid that the ciliary muscle can no longer exert adequate force on the lens to change its shape.
Loss of accommodation ability results in blurred near vision, or presbyopia. This can occur even in the presence of good distance vision. As the lens hardens, it also becomes opaque and becomes a cataract. Many people have presbyopia and cataracts simultaneously. While vision can temporarily be improved with glasses, eventually cataract surgery is required to restore vision.
If you are nearsighted (myopic), you see poorly at distance and see better at near.
If you are farsighted (hyperopic), you see well at all distances when young. As you age, near vision decreases, followed by a decrease in both near and distance vision.
Astigmatism reduces vision at all distances.
To learn more about presbyopia, watch the Presbyopia video.