Board Thread:Questions and Answers/@comment-124.106.165.201-20130206083740/@comment-6228923-20130309072507

Ouka was three years old during the Raggs War, and since the Raggs War occurred ten years before the present timeline, we can assume that Ouka is thirteen years old now. Teito is fifteen years old, thus, Ouka is younger than him.

We know that Ouka was three years old during the Raggs War because in the manga chapter where her original body died (either chapter 85 or 86, I can't remember right now) some scientists mentioned that she was only three years old.

I hope I have helped.