User blog comment:Thailog/Ep. 204 "Salvage" discussion/@comment-5069635-20120523073428/@comment-4928331-20120523104243

I take aging on the inside to mean that Superboy's internal organs - his hearts, kidney, spleen, lungs, etc. are aging. So he's not immortal - sixty years from now, he might still look like a 16-year old, but his organs are all over 70 years old.

Also, I think it's likely he is unable to age because he is a Kryptonian/Human hybrid. Unlike the fully-human Harper clones,