The fact that Fai has an unnaturally long lifespan was mentioned, but nothing was said about Fai's actual age, or that he was the oldest member of the group. Leastways, before the apparent confusion between the two possible interpretations of one of Yuuko's lines (I've seen it translated as both "Kurogane, your life is longer now too" and "[Fai] has already lived quite a bit longer than you, Kurogane.") it's difficult to say. However, just because he HAS a long lifespan doesn't automatically make him around 100.
While I agree it's more likely for Fai to have a three-digit age (if it was mentioned, he probably does) but Yuuko never said he'd lived long enough to really reap the benefits of an expanded life. He could very well be 24 and expecting to live to 1000 or something like that. However, I think that Fai's age, when one considers his lifespan, is probably around the equivalent to Kurogane's.
((And before chapter 126, I always pictured Fai to be in his 20s. He doesn't look old enough to be over thirty, IMO.))