Presbyopia: It happens to everyone over 45 years old (give or take a couple years).
There’s a moving part in our eye called the crystalline lens. The lens is clear, about the shape of an M&M candy, and sits behind the colored part of our eye (the iris). Our lens flexes when we change focus from distant objects to near objects. In our mid-40s, this lens decreases in flexibility to the point that we have increasing difficulty seeing near objects. When this happens, glasses with bifocals help us focus on near objects, such as a book or computer.