Hmm...for some reason, I always though of Sakura and Syaoran being 15-16, so I guess I was right! Yuuko, I think she has to be around 30....
Fai and Kurogane? For some reason, I always pictured Fai being just older than Kurogane, maybe by one or two years.....and they can't be too old, or still in their teens....
I'm going with Kurogane being the same age as Seishirou in TB...25. And Fai being either 26 or 27.