I'm among those who think she actually spend years in the Land of the Summoned Monsters and grew up naturally. Sure, she can still be bratty at times, but I do not think she is that bad considering she had no human role model around while she grew up. Plus, it would explain how she managed to learn a few more black magic spells, lost the ability to use white magic over time, and acquire a few more summons.

As for why Cecil and friends don't age when they go there... time flows differently. Hours (days?) they might have spent in the caves and the Land of Summoned Monsters could have just been a minute in the outside world for all we know.

I think there's no real answer set in stone for that FFIV mystery. It's up to everyone's imagination, I'd say.