Knowing what I know about why the emperor’s wife’s age is important, I can see that they’re trying to build it up into a mystery that we will slowly unravel, but without that knowledge it definitely seems like just a running gag

I have no knowledge outside what’s been shown in the episodes, but I think it’s fairly obvious that the age is some specific condition from his curse (or similar thing). At the same time, the age gap is definitely pandering to a specific audience. The author knows exactly what they’re doing, it doesn’t matter if there is some in-story pretense. For better or worse, this is the story they wanted to tell.

Jill did establish that their relationship should be platonic, which is nice. Hopefully that’s not just a front. The fact that the Jill is mentally older does help make it a little less gross (but at the same time, it feels like the author is trying to make an excuse for degeneracy).