It is a condition due to age (after age 45) of progressive decrease ability to focus on near objects.
The eye’s accommodation decrease with aging. The crystalline lens became more rigid, and it is unable to vary the curvature of its surfaces.
Presbyopia occurs as difficulty to focus closely, because the eye’s near point move away around 25-30 cm, which is also the optimal distance for close work. For this reason there are difficulties in reading and a person is naturally inclined to move farther an object to see it clearly.