Why do older peoples vison deteriorate?

The eye is able to view nearby and far off objects with ease because of the action of ciliary muscles which adjust the focal length of the eye thereby adjusting the power so that the image is always formed on the retina.

This ability of the eye is called power of accommodation.

Ciliary muscles are made of protein and as we get older, the muscles begin to weaken.

If the muscles are weak they wont function properly resulting in a fixed or slightly variable focal length of the eye. The eye begins to loose its power of accommodation.

That's why older people need spectacles and cannot see properly without them.