I think a younger villain could be interesting. There's potential for Lex to not be seen as a threat, because of his youth, but to then (of course) turn out to be the devil in disguise. (Sure, said plot point wouldn't surprise audiences one bit, but it might fool the citizens of Metropolis at least.)
True, Eisenberg does look younger than Cavill, though the actors are the same age. But Eisenberg's versatile. I'm looking forward to seeing what he does with the role.
