I have never understood the argument that Faith is older than Buffy. I always assumed that Faith was around the same age as Buffy..
I think it's partly because she's all alone and she does not appear to be in school/appears to have maybe finished high school.
We really don't know that much about her family situation, and it seems generally agreed upon that it was a bad one, but still... she's able to take off for
months without anyone at all looking for her/reaching out for her... no mentions of her having obligations to finish high school... it just doesn't seem to me like she would be a minor. (Even in bad/abusive families people have bonds and can care for each other, so anyone going "oh her parents were crap, clearly, and didn't care if she was dead or alive", really need to consider what they're implying about non-ideal family units. There can be love even when it's flawed and generally insufficient, that's not excusing anyone just saying that a poor home doesn't automatically suggest that no one cares if your troubled minor disappears/takes off)
I'm not saying that Faith is older, but I remember that when I first watched the show I always assumed that she was (if only a year).
Now my problem with the Cruciamentum is that it appears to be so age-tied. Which would make sense if ALL Slayers were called at the same age - say 15 - and then needed to be 'tested' at the same time to see if they were still sharp after three years of heavy duty Slaying (though still being up and kicking after three years seems to imply that on itself).
It would make more sense if the Cruciamentum was time-tied, i.e. say three years after you've been called rather than specifically on your 18th birthday.
As for Faith, I don't think she had to undergo it, either because of a lack of a Watcher, lack of an appropriate birthdate or just because she was under the radar by then - overshadowed by everything going on in Sunnydale and later by her turning evil and even later yet by her being in a coma.