I do think it's true that authors' outputs do often decline as they age, but only the ones who are not great writers. I think that just as the works of the very great writers stand the test of time decades and centuries and millennia later, I think likewise the great authors as well generally stand the test of time as they age and produce new and excellent work into advanced years.

I do think most creators, even the really great ones, do peak eventually, though, so there some bare merit to his sweeping claim. There are two variables I think that are worth considering here though. The first is the relative comfort level of the author. My guess is that fame and wealth do addle you, and eventually one stops producing their best work, which often requires a pain of effort that is very difficult to muster up. The elder authors that have not been lavishly rewarded I bet will end up contributing better work at their advanced age then when they were younger. The problem today of course is that most creators approaching that level of talent are recognized and rewarded fairly quickly.

The second factor, which I find very interesting to consider, is the difference one's field makes to one's creative life cycle. Film directors tend to peak in their 30's, actors too. Mathematicians and programmers and chess players in their 20's, if not a bit earlier, if I'm not mistaken. Philosophers are some of the most interesting, since they don't peak until they're solidly into their 50's, even 60's in a decent number of cases. I'm not sure about poets and literature authors, though. Offhand, excepting the truly great, does any time frame come to mind?