A young Matt Smith from Doctor Who/The Crown/ Pride and Prejudice and Zombies would be perfect, although unfortunately the real Matt’s too old now. Of course, he’s British, but I assume that all British actors can do an excellent American accent! A lesser known British actor named Russell Tovey also has the right energy for Simon (he was in Angels in America so he can definitely do an American accent). But again, too old.
One of the problems with fantasizing about casting a very young role as a writer or reader is by the time a book is optioned and all the financing is taken care of for the production, most of the actors that would seem right for the role have aged out of the part!
Somewhat less self-indulgently (don’t want to make this post all about me), the number of books that aren’t made into films (horse and otherwise) surprises me, when they have such great stories and hundreds of reviews on Amazon. Yet some very mediocre stories do get made into films–or Hollywood just remakes everything that doesn’t need a remake!