Much like adjusting the focus on a camera, in a healthy eye, the lens flexes and changes shape to properly focus on objects at different distances. However, as your eyes age, they lose their ability to flex. The result is diminished up-close vision. This is called presbyopia.
Presbyopia is a natural part of aging. Patients who have had LASIK earlier in their lives should know that their treatment isn't wearing off, but their crystaline lens isn't allowing them to continue to see images up-close the same way they used to.