When you are young, the lens of the eye is flexible. It can quickly change shape depending on whether the light rays entering the eye are coming from a distant object or a nearby one. This is called accommodation. However, over time, due to the elasticity of the lens decreasing, it cannot adapt far enough or quickly enough to be able to focus. This can have two results: close-up vision becomes blurry, and it becomes difficult to shift focus quickly between objects that are nearby and objects that are distant.
Those who experience symptoms from presbyopia tend to hold reading materials further away from their eyes than normal, and they tend to squint to be able to read clearly.
Presbyopia is sometimes known as age-related farsightedness. Another factor contributing to presbyopia is that the proteins present in your lenses decrease, so the lens stiffens so much that it cannot flex with your eye muscles, making it harder to see clearly nearby.
Presbyopia can co-occur with emmetropia (no refractive error, no prescription needed), with myopia (short-sightedness) or with hyperopia (farsightedness). If you are myopic, becoming presbyopic may make you want to remove your glasses to read. If you are hyperopic, presbyopia may make your condition appear worse.