OMG, maybe Faith feels older because she is older?
Again, age has no importance to when slayers are called. You cannot argue that Faith can't be older than Buffy because she was called after her. Because of various reasons and facts:
As seen in season 7 and in the Fray comics, there are lots of "Potentials" running around but only one of them can be called. In Fray's case, no one had been called for yeeeeears. And the potentials in season 7 vary in age, not are all 15. Faith was once a potential, as was Buffy and Kendra, though of different ages.
For some reason, Buffy got called first. Could be that destiny (or the big mystery that is the slayer line) chose her first? Probably. Then Buffy died, then Kendra died and there was Faith in line. I don't believe the counsin said "Oh darn, this girl is 18, we can't call her".
And I do dare to claime that Faith began training before she was called, just like Kendra. Having all them evil monsters and vampires spreading faster than the plauge around the world, the counsil would like to be prepared, just in case.
Then we have slayers like Nikki Wood, we can't know how long she had been slaying when she was killed by Spike or when she appears in 'Tales of the Slayers', but it has been stated more than once that Buffy lived unusually long for being a slayer, maybe because her friends worked some dark mojo on her to bring her back from the dead, but still...
Nikki doesn't exactly look like a teenager, and she's dating a cop. Unless he's a total perv he wouldn't exactly go for jailbait. Oh and Robin is nowhere to see, could be that he wasn't even born in 'Tales of the Slayer'. Which makes Nikki kinda old.
Fray is another slayer that didn't look or "seem" like a 15 yearold when she was called. And her watcher, and that demon (gah can't remember his name) had been looking for her for a while before she was finally called. That could also be the case of a lot of slayers, perhaps Faith?
Probably, the counsil wanted to call pontential slayers in their teens (I doubt they called girls in their 20's), but that's all between 13 and 19. And Faith is only a couple of months older than Buffy. She seems older and is older. Amen.