Can we simply blame the obvious change of character features and graphics between KH1 to KH2? Because while they obviously grew, when you compare SRK's faces to their future counterparts, I doubt puberty would change so much from wide, round, and disproportional 10 year olds, to actually looking like teenagers, despite that it's one year apart. Hell, when you compare Larxene's face in Re: CoM, to her Days render, or even KH2, and it's said she doesn't age, you have to wonder how her visual look got sharper.
On the other hand, I don't like the whole previous character graphics representing the younger SRK. Or Naminé. It makes it much more confusing, especially when you consider Roxas in the equation. It would have been better to simply keep it that they made the KH2 changes, and that's what they've always been. Just game graphics getting a well needed update and more realistic polygons.
Well needed.