Presbyopia is the optical term for a person who needs reading glasses. It is a condition that will affect most, if not all of us, eventually. You are likely to start noticing a change in your eyesight at around 40+ years of age.
Behind your pupil is a lens called the Crystalline Lens. This lens is responsible for how your eye operates. When you want to look at something that is close up, the muscles on the eye pushes on the lens making it rounder and, effectively, turning your eye into a magnifier.
Over time, the eye starts to lose this accommodating ability as the Crystalline lens becomes stiffer. When this happens you will start to notice that you have to hold objects further and further away from yourself to be able to see it clearer.
You may experience eye strain or blurred vision when reading books for long periods, especially in low lights.