I know that one book gave Faith's birth year as 1980, but that makes pretty much no sense. That would make her older than Buffy. Slayers are usually called at about fifteen years old, and Faith was called approximately two years after Buffy. Buffy also called Faith her "little sister slayer" or something along those lines. And the next point doesn't matter much in the world of acting, but Sarah Michelle Gellar is older than Eliza Dushku.

If you take the 15 years as a hard fact into account, you’d have to square with the fact that Willow is a full 8 years older than Kennedy and yet we’ve got them kissing in Season Seven, a 15 year old and a 23 year old, on national syndicated television nonetheless, something I’m not sure executives would have been fine on even with a straight couple. However, our articles have little to go on as far as Kennedy’s age. Other things to take into consideration are these: When would Potential Slayer-ship expire? Would you no longer be a Potential if you reached 21 and the previous Slayer dies? We also have no clue as to how the next Slayer is chosen, they seem to be chosen in happenstance, randomly or as fate or the Powers That Be decide. It’s something I think can’t really be discussed, so we’ll have to take our information as is.PonyEnglish 21:05, 11 February 2009 (UTC)
As far as Kennedy goes, it's all but explicitly stated she's at the upper end of the range to be called (Showtime: "In fact, the younger the better. In fact, I'm starting to think it might be too late for me."). So it's more likely she's 17 or 18 than 15.
As for Faith goes, if she really was born in December, 1980 (and I don't think that date is canon), that only makes her a month older than Buffy. That wouldn't put her out of the 'callable' range two years after Buffy got the nod. Both were still under 18, in any event. We could remove the date for questionable canonicity, of course. We'd could just say her age is close to Buffy's or some such.
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