Ouka is 16: http://www.mangahere.com/manga/07_ghost/c080/11.html. She was three years old during the Raggs War.
Teito was four years old near the end of the Raggs War: http://www.mangahere.com/manga/07_ghost/c028/20.html. So Teito is only a few months older than her. :)
but they have same birthdate. If Teito is older than Ouka, he must be somewhere at his 16-17 years old
What do you think?