Cranston is great actor, but Luthor at least in this version needs to be closer to Clark's age so they it could have weight to their rivalry. They have to be equal in age, which is why Corey Stoll is the best choice for this part IMO.
Why do they "have to be" anything? There's no single correct way to approach Superman or Lex Luthor. Sure, if you want to do the particular version of the story where they were childhood friends in Smallville, then it makes sense for them to be close in age -- though it's worth noting that Smallville
's Lex was portrayed as about 7 years older than Clark. But there have been a number of successful versions of Superman
in which Lex has already been an older, established businessman (or supercriminal, in earlier versions) at the time Clark begins his career as Superman -- for instance, the post-Crisis comics continuity and Superman: The Animated Series
Besides, we've already seen a Lexcorp logo on some trucks in Man of Steel
, which seems to confirm that they are indeed going that route. So casting an older actor as Lex is necessary to be consistent with what they've already done (unless they wanted to copy Smallville
and use Lionel Luthor, which is unlikely).
I like the idea of Luthor and Superman being opposites in many ways.
Luthor: Older, rich, amoral and a "thinker".
Superman: Young, humble origins, moral and more physical.