What causes presbyopia?

Lens gets too stiff. So it can't focus light on the retina as objects get closer.
Aging. Presbyopia is the loss of near reading ability due to loss of accommodation of your lens/lens zonules in your eye associated with aging requiring use of reading correction. This starts approximately 40 years old.

Related Questions

What can cause presbyopia?

Living over 40. This is a natural age related hardening of the lens of the eye. It cannot be prevented by any currently known method (just as grey hair cannot be prevented). Your ophthalmologist can help you with advice and devices to compensate. Read more...