As we age, the lenses in the eyes lose some of their elasticity. Without this elasticity, the lens is less capable of changing focus for different distances.
Presbyopia may seem to occur suddenly, but the actual loss of flexibility takes place over several years. Long before an individual is aware that seeing close up is becoming more difficult, the lenses in the eyes have begun losing their ability to flatten and thicken. Only when the loss of elasticity impairs vision to a noticeable degree is the change recognized.
Presbyopia usually becomes noticeable in the early to mid-forties.