I love that explanation. I think you're right--the younger look is more in style right now, and movies play off it. Being older and wiser just doesn't hold as well. Maybe it also is used as a way of making an additional excuse for the hero when he makes a mistake as well. An older Superman probably couldn't get away with something a younger version would (look at all the crap Clark in Smallville does, as example). Routh does look younger--he's not really handsome as much as cute, and I can see why some fans aren't happy with that.
However, as a girl in her mid-twenties...um, I have NO problems with Routh's appearance.