As we age, the lens of the eye becomes increasingly inflexible, making it harder to focus clearly on near objects. This is called presbyopia. No one knows exactly what causes the lens to become inflexible, but it happens to everyone as a natural part of aging.
In order for us to see images clearly, light rays enter the eye, where the lens bends and focuses the rays on the retina. The lens changes shape to allow the eye to focus on objects at different distances. Beginning early in life – perhaps as early as age 10 – our lenses gradually stiffen and begin to lose the ability to change shape. By the time we are in our 40s, the lens has trouble focusing up close, and we begin to experience blurred vision when we try to do tasks that require up-close focus, such as reading or needlework. The lens continues to stiffen until about age 65, when nearly all its flexibility has been lost.
Presbyopia eventually affects everyone, even people who are already farsighted (hyperopic) or nearsighted (myopic). Because people who are farsighted already need to work when focusing on near objects, they may experience presbyopia a little earlier in life. People who are nearsighted will still experience presbyopia when wearing their corrective glasses or contact lenses for distance. However, for looking at objects or letters closer to them, they will likely be able to see quite clearly when they take off their glasses. This is where the term "nearsighted" comes from.