Presbyopia occurs as a result of the following
Shoening of eye ball
Hardening of lens
Thinning of the lens
Lengthening of eye ball
Presbyopia (eyesight of old age) is not an error of refraction but a condition of physiological insufficiency of accommodation leading to a progressive fall in near vision. The cause of presbyopia is believed to be due to thickening and loss of flexibility of the lens in the eye. Reference: AK khurana 7th edition
Optics and refraction