Wow. I lurk the forums a lot, but rarely ever post... yet this deserves my input:
Zexion. Right there is your paradox. It's not just his shoes on his model between the games. It goes deeper than that. It's how long he's been a Nobody that's the important factor (and this can be applied to any of them, really, but Zexion the age gap is most notable: Ienzo, if you read the reports, was the youngest apprentice).
In KH1 (and also the manga - *thumbs up*), Squall/Leon says to you he and the others fled Hallow Bastion seven years previously. (Need I mention Birth by Sleep takes place ten years prior to KH1? Fitting.) They fled because the world was on the verge of collapsing - too many Heartless running around? I'd say it was about this time that the apprentices, who were behind the world's destruction, inevitably, lost their hearts and wound up as Nobodies.
I don't care what age you think Zexion to be in Chain of Memories. Whatever it is, subtract seven years from it. Do you still think he'd be an apprentice? If you make him 20 in CoM, suddenly he was a 13-year-old apprentice to Ansem the Wise. Doesn't make much sense. That concludes my input on why I think Nobodies don't age. /end rant