I never find there is a research associated with such questions. And there is no evidence that proves whether sunglasses lose UV protection over time. But personally, I heard the sunglasses can prevent UV rays because of the UV coating on the lenses. So, i believe that the sunglasses won't lose UV protection unless you damaged their UV coating.
Maybe. More than 90% ultraviolet rays will be filtered by the coating film of sunglasses. But this kind of function will be deduced if the films suffer by scratching. So coating film can not be touched by organic solvent or hard objects to protect the coating.
I never heard that sunglasses will lose UV protection over the time. People buy new sunglasses usually because of fashion and tired of wearing old sunglasses. Still some people buy new sunglasses because their old sunglasses broken. But i never met people buy sunglasses because their old sunglasses lose protection. So, you really needn't worry about the questions.