Freddy Fermin has been way, way better than I think anyone other than maybe his mom expected this season. He is a bit unusual though, so I am not sure what we should expect from him going forward. To try and figure that out, I have combed through some history and his advanced metrics to see what it is the Royals have found in their Venezuelan backstop.
Most players in the pros make it to their rookie year before they are 28, if they are going to make it at all that is. I pulled every position player rookie since 1990 to get at least 250 plate appearances (about what Fermin will end up at), and isolated all of them that were 28 or older in that season.