Why do most people over the age of 40 need reading glasses?
In order to focus on objects up close, the lens inside the eye needs to change its shape to become fatter and bring the light rays into focus on the retina. As the eye ages, the lens becomes more brittle and less flexible. As such, the distance to which we can bring objects in and still keep them in focus drops off as the lens becomes more and more brittle and less and less able to change its shape with increasing age. Reading spectacles focus the light at the retina and make the image sharp again.