So, Returned get the Fifth Heightening by virtue of being Returned, right? It's explained in the book that, among other things, the Fifth Heightening is what grants you agelessness. (In fact, that's the only new ability the Fifth grants you.) So that's why the gods don't age; makes sense.
However...Vasher, despite being Returned, is suppressing most of his abilities, including those that go along with the Heightenings. In the
annotations for chapter 22, we find out that Vasher (and Denth) do still have certain physical enhancements, like speed, even when they're suppressing their Returned nature. However, I don't see why agelessness would be one of these, unless you
also get it just by being Returned, and not only by virtue of reaching the Fifth Heightening.
Does anyone have any plausible explanations, or (even better) know anywhere that Sanderson has addressed why Vasher, Denth, and the others are ageless despite suppressing their Returned Breath (and the accompanying Fifth Heightening)?