Its nothing more than a problem associated with eye lens and eye sight as we age up. don't you know that our old aged grand parents find it very difficult to read and see, the main reason behind it is presbyopia. the crystalline lens of your eye loses its flexibility. as you get old your lens can no longer change shape and so cannot see close objects and this condition is presbyopia.
P.S: speciality of our lens is that it can change shape according to focus on distant or nearer objects-short words power of accommodation. you'll lose it in the case of presbyopia.