why do people have less vision when old? eye lens are reasons?

Answer 1

Yes, the lens is one reason for imperfect vision that occurs with increased age. The elastic capsule of the lens becomes less elastic with age, and is thus less able to round up for fine-focusing -- this change is called presbyopia.

Cataracts are protein accumulations that also may occur with increased age, and also are found within the lens. These may additionally contribute to imperfections of vision over time.

Answer 2

the lens in your eyes are operated by tiny muscles. just like any muscle in your body, it needs to be used often to keep its flexibility and strength. But, muscles, no matter how much you exercise, will get weaker and weaker with age. same thing with your eye muscles. There are also cumulative reasons, like a slow build-up of cataracts which only become serious after long years (you'll be old by the time it affects your vision).