Step 1: Recall what changes as a person ages.
With increasing age the ciliary muscles that control the eye lens grow weaker, and the lens itself becomes less flexible, so the eye slowly loses its power of accommodation.
Step 2: See what this loss does to vision.
This condition, called presbyopia, often leaves a person struggling with near vision for reading, while sometimes also carrying an older, separate difficulty with distant vision.
Step 3: See why a single lens will not fix it.
A person who needs help with both near and far vision cannot be corrected with one uniform lens power, they effectively need two different corrections at once.
Step 4: Connect this to bifocal lenses.
A bifocal lens solves exactly this problem, its upper part is ground for distant vision and the lower part for near vision, letting a person read and see far through the same pair of glasses.
\[ \boxed{\text{presbyopia}} \]