I don't care how much Negi has accomplished in the course of the story, you just don't accumulate the kind of power and resources Chao had by age 15. There's no freaking way she's actually a young teen -- even in a world with, ahem, negotiable realism like that in the manga. And given that we know of at least one method for reliably and controllably altering one's physical age, I still maintain that she must be older than she seems.
-- Bob
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Then the horns kicked in...
...and my shoes began to squeak.
-- Bob
---------
Then the horns kicked in...
...and my shoes began to squeak.