Namine is the exception that proves the rule. She ages, yes, quite clearly.
But Roxas does not. We know for a fact that the exact same model is used from the time of his birth. He doesn't grow, he doesn't even physically change. Plus, why would things that don't exist age anyway?
As for the explanation behind Namine, no one knows. It could very well be that because of her lack of a body and soul, the nature of her "existence" is different in that she ages with Kairi. How this is possible, I don't know, I don't think it matters.
And anyone who brings up how Zexion has different shoe sizes? Re:CoM has various subtle changes in models and shit for whatever reason. It might simply be that the Re:CoM developers used newly created models of the CoM Organization members, and, in doing so, made slight modifications like with his shoes. On the other hand, maybe KH2FM might have used the slightly modified versions of the character models that they already had in KH2.
Either way, we know that Re:CoM does some things different, as seen with, for instance, The Place Where Nothing Gathers:
YouTube - Kingdom Hearts Re: Chain of Memories || After Sora's Credits
YouTube - KH2FM - The Plans (NEW SCENE)
(Notice how in Re:CoM, the area seems more "misty" as reflected by the darker tones of their coats)
So I don't think something as subtle as shoe variation reflects age... in this case.
In other words, I'm overall on the "Nobodies don't age" bandwagon.