The lens is made of mostly water and protein. The protein is arranged in a precise way that keeps the lens clear and lets light pass through it. But as we age, some of the protein may clump together and start to cloud a small area of the lens. This is a cataract. Over time, the cataract may grow larger and cloud more of the lens, making it harder to see.
Researchers suspect that there are several causes of cataract, such as smoking and diabetes.
Or, it may be that the protein in the lens just changes from the wear and tear it takes over the years. You don't have to be a senior citizen to get this type of cataract. People can have an age-related cataract in their 40s and 5Os. But during middle age, most cataracts are small and do not affect vision. It is after age 60 that most cataracts steal vision.