The lens inside the eye is soft, and muscles surrounding it cause it to change shape in order to change your focus between near and far.
When you are over the age of 40, the lens hardens and wont change shape so easily. This results in an inability to focus of close objects without reading glasses and is known as presbyopia.
In older age, the lens can start to change colour, and cloudy patches can form in the lens. These cloudy patches are known as cataract.