A cataract is a loss in the transparency of the crystalline lens within the eye. Although they can occur in children and in young adults with ocular disease, they most commonly occur as patients get older. The ageing process in the lens results in it stiffening and losing flexibility (hence most people need reading glasses after the age of 45). Cataracts are really just an extension of the same process so that the crystalline lens first loses it’s flexibility and then becomes opaque.
There are various types of lens opacification and the degree to which the vision is affected depends on whether this opacification is homogenous or piecemeal in character. Little bits of opacification here and there within the lens structure cause more problems with your vision than homogeneous changes, the effect of which is more akin to wearing sunglasses. Some people with quite marked cataract have relatively little impairment of vision. These sorts of cataracts (often termed nuclear sclerosis) are more commonly found in elderly patients. On the other hand, a small amount of opacity right at the back of the crystalline lens (a posterior subcapsular cataract) can cause a lot of problems with sight, particularly glare driving at night. These changes are frequently seen in younger patients (40-60 years old) with cataracts.
Click here for more information from the American Academy of Ophthalmology website