In the book the Emperor is described as being in his early seventies yet looks no older than 35.

I hoped Villeneuve would finally cast an actor in his late thirties to early forties or least one who looks deceptively young (Jared Leto comes to mind, as he’s 50 years old but can easily pass for a much younger man) so we could have a book-accurate Emperor Shaddam IV. Alas, Villeneuve has chosen to continue the tradition of casting much older actors in the role.

IMO Walken would’ve been much better suited to play Count Fenring.