As our bodies age we generally notice bits of us don’t bend as well, and muscles don’t work as well. The eye is no exception after the age of 40 – 45 the lens begins to become less pliable and the cillary muscles are less strong resulting in an inability to pull the lens into a more powerful shape. This in turn means the eye can no longer pull distance focus back to reading distances.
It can be corrected by using reading spectacles, bifocals or progressives.