Authors improve with age? In my experience that's not true at all. There seems to be a range during which authors are at their optimum and even if the actual age range varies from person to person. The consistency is how the decline manifests itself.
Too many authors shift from storytelling to exposition in their later years. Instead of describing a compelling narrative into which thought provoking concepts are intertwined they get totally fixated on those themes. So you get a book full of exposition in which virtually nothing happens until the very end; it's a book full of people talking instead of doing. It seems exacerbated by sticking to the same universe but I've seen it happen with unrelated novels by the same author.
I always bring up Frank Herbert and the Dune series as a case study for this phenomenon. It's not that there aren't facets of the later books that aren't interesting, but as a novel those later novels are not as engaging as the first, even when they had the potential to be so much more. And it seems that first novel is usually the best.