It's been confirmed that Syao Syao and Sakura are 15 in Tsubasa? When was that confirmed? (It's not that I don't believe you guys, I'm just curious...) Anyway, I think that age works for them, but like someone else said I DO think they're a little short for 15, but CLAMP is screwy with height, too! XD Also, both Kuro and Fai have been described as "tall" so maybe Syao and Sakura are average height and K/F are tall! XD But I guess that doesn't work because S/S are short around other people, too..
Anyway, I actually agree with the people who say Fai may be older than Kurorin.. I don't know why. In the beginning, I always figured Fai would be younger, but as the series went on, he just kept seemer wiser and wiser and more and more mysterious, which somehow makes him seem older to me. He also definitely seems to be the most mature of the group (Except for when he's playing with Kuro, of course..) because he always seems to understand things faster than everyone else and he's good at listening to people. He also seems like he's been through a lot, himself, so I'd think he'd have to be older to have gone through so much already.
My guesses: Kuro and Fai are mid to late 20s, and Yuuko-sama is around thirty. (Because she seems even older than everyone else, but she sure doesn't look like it!)