Subtle effects of old age

My father has been a great reader. He enjoyed books and spent a lot of time with them - until recent years, that is. He used to be a big fan of Louis Lamour westeren novels, but I noticed that he left off reading even those some time back. He said he just did not have an interest in them any more. That puzzled me a little until I realized that his short-term memory has deteriorated to the point that he cannot remember the plot from the first of the book to the last, so it does not make any sense to him any more.