Presbyopia

Presbyopia is when the eye begins to loose the ability to change focus from distance to near.  It is not an eye disease, but a normal age related process.
In a young healthy eye, the muscles inside the eye act on the lens, changing the shape of the lens and allowing clear vision distance and near.  However with age, the flexibility of the lens decreases and near work becomes more blurred.
Generally presbyopia becomes noticeable in the early to mid 40's.
There are a number of different options to correct presbyopia, including reading glasses, progressive glasses, bifocal glasses and multifocal contact lenses.