In a young person, the natural lens of the eye is soft, flexible and clear. This allows the lens to alter its shape and optical power in order to shift focus and maintain clear vision over a wide range between distance and near. As the lens becomes gradually stiffer with age, it less able to change focus. This happens even if you can still see clearly for distance while wearing glasses.
For many years, it goes unnoticed because few things need to be viewed very closely. Most people start to notice the loss of focus power by their mid-forties, when they need to hold reading material further away to see clearly or find it easier to use reading glasses. At this stage, the eye is said to have developed presbyopia.
Although changes in the lens occur at much the same age in everybody, the effect on vision may be noticed at a different age, depending on whether the eye is long-sighted or short-sighted to begin with. People who are long-sighted develop difficulty reading at a younger than usual age and may need reading glasses in their thirties or earlier. Those who are short-sighted develop difficulty reading at a later than usual age and may not need reading glasses until their fifties, or even later. People with a low level of short-sightedness may remove their glasses to read or move them lower on the nose.
The strength of reading glasses is normally given by a figure such as “+1.50” to indicate that the focus power of the eye needs to be increased by that amount in order to see clearly for near. If you already wear glasses for distance, the additional focus power required for reading is referred to as the “add”.
No surgery can reverse this aging process but surgical and non-surgical techniques are available which may reduce the impact of these changes on vision.