Well... maybe so, but he's almost 30, which is six years older than Reeve was when he first donned the tights LOL.
No doubt, Welling has features that give him a youthful appearance (large eyes, short chin, etc.) but they accentuate it with make-up and hair style. They make his eyes look even bigger with eyeliner (hard to believe), comb his hair down over his forehead (bangs make a HUUUUUGE difference), give him lipstick that no man should ever have to wear (LOL), and paint him with enough foundation to permanently seal up every last pore on his face. At the end of the day, he probably peels off his make-up like a mask. Poor guy. It's no wonder he lets his hair and beard grow during hiatuses.
Basically, he looks young because he's supposed to. But his actual age squeaks through when he's not made up, and sometimes even on extreme close-ups...
Given today's CGI effects and practical make-up knowledge and ability, actual age is really a non-issue. I look more at things like bone structure and eyes, 'cause while they *can* change that stuff, I'd rather they didn't. The very features that give Welling such a different look can be used to drive home the preternatural alien qualities that should be inherent in Superman - and as fate would have it, Welling has those features in spades - starting with his face and eyes...
BTW, if I didn't already say, welcome to the Hype, Ariellem.