Mental decline and brain ageing happens to each one of us, including the most intelligent and super sharp brains. You will be surprised to know that some of the great intellectuals of the world such as the famous noble prize winning writer, Erich Segal suffered from Parkinsin's disease through most of his life. There are other notable cases of famous scientists and intellectual geniuses with serious mental decline disorders.
So this makes clear that anyone can suffer from mental decline and mental disease. The main reason behind brain ageing and progressive decline of cognitive abilities overtime is the free radical damage and the process of glycation. It is these two processes which have been found to be main causative factor behind Alzheimer's disease. Free radical damage is a process in which important brain cells get damaged due to oxidative stress.