Theory #1: Ben's Ghost Ages Because of His Connection to Klaus
Due to Klaus's ability to manifest the dead into the living world, it's entirely possible that Ben's ghost ages because of his strong connection to Klaus. While Klaus is able to send other ghosts away, Ben is always there, making Klaus an anchor of sorts to the real world. As Klaus has gotten older, his powers have gotten stronger, which could very well have allowed for a power transfer between the two. Perhaps this is why sometimes Klaus has a conscience?