Does Vocabulary Shrink with Age?

Robert Birnbaum interviewed Martin Amis, who was once quoted as saying that your vocabulary shrinks as you get older. Asked to elaborate, he replied:

It’s just a fact. Ah, there are horrible statistics to show it. (Both laugh). But I have always been a great user of dictionaries, and thesauruses as well. People get the wrong idea about a novelist and the thesaurus. You are not looking for words like “sinecure.” What you are looking for, most of the time, is a synonym that scans differently. So you need a three-syllable synonym, or one that doesn’t end in “-ation.” It’s just for the euphony. Um, but the COD [Concise Oxford Dictionary] is all you need. You look up a word, maybe a word you know perfectly well, and you see what it’s origin was, and that word is now stamped. It’s your word. And every time I do it, if feels like a gray cell has been born in my head; instead of going out, it’s coming in.