It's possible that Terra aged?

[Articus]

The Dark Realm has a slower flow of time. For her it's not been 10yrs.

From what we learned in DDD and BBS interviews each world has it's own flow of time in the Light Realm.
The Dark Realm flows slower in general and the Dream Realm has no real flow of time at all.

For REALSIES? When did this come to light? (Haha...ha)

 

[Gram]

For REALSIES? When did this come to light? (Haha...ha)

The Dark Realm bit came to light a few years back:

- At that point, it seems like Aqua has been lost for a long time.

Nomura: Yes. From when she comes to the world of darkness to when she meets Ansem the Wise, over 10 years have passed in the outside world. The flow of time is different in the world of darkness and the outside world, so it doesn't feel like 10 years have passed. But there's no mistaking that she's been in the world of darkness for quite a long time.

link: Full Famitsu Interview - News - Kingdom Hearts Insider

The part about the Dream Realm and Light Worlds was pointed out by Yen Sid and Riku during DDD. During Rikus talk with Joshua:

Riku: So time flows differently here and there? So what? That's true of
any two worlds. Their home world would be running on a different time
axis, too.

Yen Sid: In the Sleeping Worlds, real time does not flow. Unless one
restores the world by waking it from its slumber, it will stay locked in a
dream forever. Thus, you may encounter familiar faces...but they are just
figments of the dream. In actuality, they are sound asleep--trapped within
a world that is also sleeping. What's more, whilst someone may no longer
dwell in the real version of a world, dreams may paint a fuller picture,
and restore what seems to be missing.

For the realm of Light time varies between worlds, for the realm of Dark time is slow overall and for the ream of Sleep it's nonexistent because the worlds, true to their name, follow the flow of the dreams their having.