But, for those twenty years, she didn't age a single year. She was admitted into Mahora, at an (apparently) early age, and has aged like a normal teenager til now.
I don't think even those of Ala Rubra could do something like that.
I would be inclined to suspect Chao and her time machines had something to do with it, since it was "roughly 20 years ago", meaning the World Tree's power might have been active at that time (Once every twenty two years, but the last one was activated in just 21 years), but I believe that would be too easy an explanation.