One idea that I had that I felt would "fit" even though there is no direct evidence for it in the game is that the process of becoming a nobody can cause the body to rapidly age in accordance with how strong the heart is. If the body rapidly ages beyond it's normal "death" time, it becomes a non-human nobody.
So weak hearts that are strong enough to form a nobody=the body is so malformed that it no longer looks human, instead is that white dusk/thing.
Medium hearts=Ienzo, ages a bit but retains a human form, same with Lea and Isa, also Xigbar, who DOES look older than Braig.
Strong Hearts= Dilan and Elaeus, hardly age.
Super-Strong=Xehanort, Kairi, and Sora, who's nobodies would not age at all.
AFTER that process happens, their age is frozen.
One might ask then, how does Xemnas look older than Terra, why does Namine age?
What "fits" with that imo, is that Namine aged because her heart wasn't consumed by darkness, Kairi was still growing normally, so Namine also grew.
Same thing with Xehanort, Ansem SoD was an active, sentient semi-heartless, not a completely consumed feral heartless. Thus Xemnas would also age.
