I was thinking a lot of this character particualry, first of the points that some one mention is the "age thing", we have to remember that CLAMP like a lot to desguise the age, we have to remember Magic Knight Reyearth, maybe Fai is a lot older that we thought....
And we have to think about his world, a lot of magic, there is a past, very hide... other think we have to remember the relationship with Ashura-ou & Fai (remember the part of the manga, chapter 130, when he said to Yuuko "a lot of lies, an when Ashura.ou took him in"), maybe The past of Fai is more related with the plot of Magic Knight Reyearth; this is my confusing conclution, guys.... i don't know, we have to analize all the CLAMP work and "try" to think about this character in particular....