Let's not lose sight of the fact that, back in the '50's and '60's, Superman looked middle-aged (around 40-ish?) because he was written, drawn, and edited by middle-aged men, who, perhaps, put more than a bit of themselves into their interpretation of the character.  A character who was, after all, regarded by many of his young readers as something of a father-figure.  Growing up, I always thought Curt Swan's Superman looked at least a decade younger than Wayne Boring's, and a good half-decade younger than Al Plastino's. 

E. Nelson Bridwell once wrote in a text page in a Lois Lane 80-Page Giant that Lois and Lana were both "about 30", and Superman was "about 32"; this was before the hippies started their "don't trust anyone over 30" mentality, and women started panicking about 30 being "over the hill".