As people get older, their eyes are less able to "accommodate" (change to see both far and near things). The eye muscles are at their most relaxed when you look at distant things, but when you look at nearby things, the muscles of the eye have to contract to bend the lens of the eye so you can see the nearby thing clearly. That's not a problem when you are young and the lens of your eye is flexible, but the lens keeps growing thicker throughout life and gets stiffer and harder to bend. That can lead to eyestrain as your muscles get tired. That's why around middle age, people with good far vision start needing reading glasses for close work like reading and computer time.