Apparently, Tom Welling, who plays teen Clark, is older than Brandon Routh who played adult Clark.
Well, Clark has been out of high school for a few seasons now. And Brandon Routh was too young for the part, in my opinion.
On the subject of Smallville, I only took an interest in the show a couple of years ago. I stayed away for the reasons Pennywise gave - no interest in pretty young stars turning Superman into a teen soap opera. I forget what originally made me want to check it out in Season 5, but I liked it. And since reruns were airing daily and I was at home with an infant who would nap for two or three hours daily, I caught up on the whole series over a couple of months. The early seasons were very much a teen drama, but the freak-of-the-week format kept it entertaining, and Welling plays a likeable Clark. During the middle seasons, it was actually really good. I thought it was an interesting take on the Superman story. I haven't cared much for it since all of the older characters have gone, however. John Glover as Lionel Luthor was a big part of what made it good. The only thing that's keeping me watching this season is that it looks like they're finally trying to wrap it all up.
What I have liked about it is that rather than portraying Clark Kent as Superman's disguise, Smallville is doing it the other way around. Clark is the real person, and the separate identity he's beginning to create is the disguise, to protect his private life when he isn't out superheroing.
My main complaint about Smallville, at least regarding the later seasons is the "no tights, no flights" rule that everybody is still treating as gospel. Clark has known for how many seasons that Kryptonians can fly? He's had no trouble embracing any of his other powers. He's shown signs of defying gravity from day one, and he's just plain flown while under mind control. He's not afraid to jump out of an airplane and freefall to the ground, or ride an ICBM into space and then reenter the atmosphere. So, why is he still refusing to fly? As far as I can tell, it's because the show's creators, for dramatic purposes, made a rule that made sense for the first few seasons, but now makes absolutely no sense in light of what has happened in eight seasons. They're rushing Doomsday into the story before Superman is even known to the public, and yet Clark isn't ready to fly? And since last season, they keep dangling the possibility that it might finally happen, and then jerking the audience around. That's getting tiresome.