Final answer:
Presbyopia is caused by the hardening of the lens in the eye, leading to difficulties in focusing on close objects as we age, often requiring the use of reading glasses or converging lenses.
Step-by-step explanation:
The part of the eye that hardens and causes presbyopia as we age is the lens. With aging, the lens becomes less flexible, making it harder to focus on close objects. This is why many people over the age of 40 may need reading glasses or corrective lenses with a converging lens to help with close vision tasks.