I guess, but that doesn't sti well with me. If they were lazy, they wouldn't have bothered to create a whole new model of Naminé for Re:CoM, and show that she aged. If Roxas had aged I'd think they would have created a new, younger looking model for Roxas in the scene where you see his birth. But they didn't, so...
And besides, Roxas looks about the same in Days as he does in KH2 if you ask me, though that's really little more than an opinion.
Also, you're forgetting that Naminé is an even bigger anomaly than Roxas is. It might not make much sense that she, the nothing, would age, and the other nobodies who have bodies and souls wouldn't, but that's what it seems like.