The lens is a flexible elastic structure, its shape can be changed by the small muscles that are attached to it. When looking at something close like in reading, the muscles contract increasing the curvature of the lens adjusting the focusing power.
Unfortunately, as a normal part of aging, the lens loses its flexibility with time and subsequently, its focusing ability, making it hard to see at a normal reading distance. This change usually starts in the forties and continue to worsen until the mid-sixties.