At the time that it was first on, I just naturally assumed that Faith was younger, though not by more than a year or so, that Buffy. Over the years since then, though, I had come to wonder. But in Season 7, in one of Buffy's speeches to the troops, she made the statement, "The younger, the better," referring to which Potential was the most likely, and Kennedy, who was almost certainly 18 or older (can't have Willow messin' around with jail bait now, can we?) made the statement that she was beginning to think that she was too old. That would seem to indicate, though we've never been told anything about how the actual Calling works, that there is a window between the ages of 15 or 16, when Buffy was Called, and 18 or 19, which would have been Kennedy's approximate age. If we accept that, and take into account that Faith is two "generations" later than Buffy, that would almost require her to be the younger of the two, by probably a year or so.
Unless, of course,and quite possibly, my calculations are of as much value and validity as a politician's promise, in which case, Faith is probably older than Buffy. Or the same age. Or younger.
Only Joss knows for sure.
