Right now he certainly looks the part. Christopher Reeve and Dean Cain may have spoiled us into viewing the character as a young, relatively slim guy, but traditionally Superman has been a barrel-chested, chisel-chinned, mature man. Artist Alex Ross has always done a great job portraying what I consider to be the iconic image of what the Man of Steel looks like. I think one of the many mistakes that Bryan Singer and the other makers of Superman Returns committed was casting Reeve-lookalike Brandon Routh in the lead. Routh is a fine actor, but it seemed he was too busy channeling Reeve rather than making the character his own.
It is understandable that the studio would want to cast a young actor to spearhead the franchise. If multiple movies are made, someone who is 40 will likely show his age after a couple of sequels. But that is a challenge that every actor must face when playing a superhero who arguably doesn't age as ordinary mortals do. Superman is invulnerable, so would his cells age at the normal rate?
The comics have handled the age factor rather well, so there is no reason to doubt that the movie series could do the same. I would rather see a story with Superman as an established figure, like in the Kingdom Come storyline, than another origin storyline with a 20-year-old who would be better suited to play Superboy instead of Superman.
Some of the best performances of heroes have come from older thespians -- Robert Downey Jr. as Iron Man, Michael Keaton as Batman, George Reeves as the original TV Superman, and others. There should be no fear that Jon Hamm or another actor in his 30s or even 40s could not succeed in playing the part of Superman for ten years or more. Let us hope that Warner Bros. and Nolan do not take the seemingly easy way out and cast a young pretty boy to play the mighty Last Son of Krypton.