Presbyopia is the cause of reading blur that occurs with age. Most people begin to notice the effects of presbyopia sometime after age 40, when they start having trouble seeing small print clearly — including text messages on their phone.
Presbyopia is natural to aging. You cannot exercise the muscle in your eye to get stronger for the purposes of improving focus. The muscle is not the problem. It is the lens in the eye that stiffens and limits focus. This happens even if you've never had a vision problem before. Even myopic people who can see very well up close without glasses will notice that their reading blurs when they wear their usual eyeglasses or contact lenses to correct distance vision.