Here's one explanation that, while not official, I believe fits such that one could consistently explain this nobodies-aging stuff.
Becoming a nobody isn't as simple as a Soul and Body(SnB) separating from the heart.
The nobody is formed by
1) The heart collapsing under darkness
2) Memories from the heart transferring to the nobody(Roxas not remembering cause Sora came back too quick, Sora also said he was forgetting things as he fell into darkness.)
3) The SnB is re-formed in a different place(CO, Twilight Town).
Note that.
-A few nobodies, notably Xigbar, Ienzo, and Xemnas, appear significantly older than their somebody counterparts.
-Those nobodies lost their hearts and formed only a year after BBS(I believe).
-There are a lot of nobodies that did NOT retain human form.
With this in mind, I believe that the process of becoming a nobody begins to mal-form the body that is to become the Nobody.
The degree of this malformation depends on the strength of the heart consumed by darkness.
In the case of a weak heart, the body malforms so much that it becomes a dusk-like nobody, losing human form.
In the case of a resilient heart, the body ages substantially. Zexion, Xigbar.
In the case of an immensly powerful heart, the body hardly ages at all. Xemnas, Roxas.
AFTER the malformation process, the nobody can no longer age.
1) Why does Namine age?
Unlike Zexion, her heart, Kairi, is aging normally, causing Namine to age.
2) Why did you list Xemnas under those who don't age when he is supposedly 30yrs old by Nomura's words?
Because I belive Xemnas's aging is like Namine's, he didn't malform-age like Zexion, but ages normally due to Ansem SoD being active and alive instead of a traditional heartless.
3) Why didn't Roxas age in days if Sora visibly aged into his KH2 appearance.
Ven's heart was in a state of dormancy in Sora, as was Ven's vessel. Since Roxas's appearance is based on Ven, he will not age unless Ven's vessel started to age as well.
How does this all connect to the Mysterious Figure?
It takes some heavy assumptions, I'm not trying to argue any of this is true, just saying that it "fits" hypothetically imo.
If a heart going into darkness causes the vessel to malform and turn into a nobody, what happens if a Heart is forced into a vessel of light?
MX's heart didn't fall into darkness, although Terra's heart was very dark, I feel safe in saying he went into a heart "lighter" than his.
You can think of that process as the opposite of becoming a nobody. Instead of collapsing into darkness, the heart finds a new vessel, and loses none of the memories.
So this reverse-process might actually cause the exact opposite to happen to the body, i.e MX's vessel became YOUNGER and reformed in another location.
Nomura did say something along the lines of the MF being a type of existence we haven't seen before.
Perhaps this new existence is essentially some kind of "reverse-nobody". How would it be different than a regular nobody other than the way it is formed? I don't know, all I know is that it is formed by a process that could be called the opposite as becoming a nobody. Instead of your heart going into darkness, it goes into a location "lighter"(terra's heart) than it had previously been, rather than a location "darker"(realm of darkness)..