Presbyopia is not truly a refractive error, but is the natural process of the lens losing its ability to accommodate, or change its shape inside the eye. Why does the eye lose its ability to focus at near? For certain, it is not a disease or degeneration of the eye. One popular theory holds that the tiny muscles inside the eye that control the lens grow tired and lose their tone with age. Exercises were tried in an attempt to strengthen these muscles. The best information we have today regarding the gradual loss of accommodation places blame on the lens itself, which lies within its own capsular casing. Unlike the rest of the body, which stops growing by the age of twenty, the lens of the eye continues to grow throughout life. As the lens ages and grows, it becomes harder in consistency and, because it is confined within that capsular casing, it is therefore resistant to changes in shape. The result is a gradual reduction in accommodation, and more dependence on reading glasses.
If you also have a cataract (cloudy lens) developing you may be a candidate for a premium IOL (intraocular lens) that can decrease your dependence on glasses.