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Chet31 Perhaps I'm wrong on this but from what I remember reading through I believe the Archduke is implied to be much older than he looks. That would also explain why Dan dismisses him as Grandpa. Moreover, he's known as a long time friend and rival of sorts for Charlotte, and they speak to each other as equals. He also treats her disciples like they're kids like she does. If he was much younger than her and closer in age with her disciples I think that would be less likely.
Edit: I just read that nehnehneji made pretty much exactly the same points I did, so you get the jist. Mind you she could go for someone younger if she wants, after all she is sort of living a second life, but she herself has stated she's uncomfortable with that idea.