I'm okay with Superman aging, slowly, in alternate takes (especially in the likes of Kingdom Come where it's symbolically significant), and I see the New 52 version as aging over the millenia for reasons I don't entirely understand myself. But generally speaking, I prefer 'classic' versions of Superman don't age that much, if at all. It's actually part of the reason I don't see the Earth Two Kal-L as 'actually' being the original Siegel/Shuster Superman: that's not just 'a' Superman, that's THE Superman, and that Superman of Action Comics #1 is energy and playfulness and power and utterly unstoppable, he'd never get old or even slow down as long as there was something left he had to do.
Mostly what Ascended said, as per usual.