EXCLUSIVE: Casting Search For Superman Begins
... specifically, that the actor will either be a discovery or on TV but likely someone who isn't well known yet. And he'll be in the age range of 28-to-32. ...
This obviously doesn't rule out Tom Welling(Smallville) & Brandon Routh(Superman Returns).
It does rule out Welling, who is 33 and well-known. And for these purposes, Routh definitely qualifies as well-known. (Ironically, Routh, who played the adult Superman, is two and a half years younger than Welling, who plays the "boy" Clark Kent.)
Matt Bomer, whom I've thought would be a good choice, is also 33 (6 months shy of Welling's age), so if those numbers are strict, it seems he'd be ruled out too.