In a middle-aged to older adult, if an eye gets more and more myopic/shortsighted over a period of time, it could be due to cataract. The mechanism for the myopia is different compared to that happening in young children.
In children, the increasing myopia is usually due to lengthening/growth of the eyeball. When a cataract develops, sometimes the focusing power of the lens of the eye increases, causing an increase of myopia as well. This is called index myopia.
There are other more uncommon causes of new myopia such as changes in the shape of the cornea. Occasionally young adults in their early twenties may also get myopia like young children where the eyeball grows longer. However, this is uncommon.
