Well, we don't know how it works at all, or if it's even the Powers that make the selection (my impression is it's not, if only because the Powers weren't a part of BtVS, and thus it's unlikely they were meant to play a role in Slayer mythology). But I would guess the Council has the best understanding of the parameters, just by virtue of studying the patterns of Slayers called.
Like I said, bad with guessing ages, but I could see Nikki being early twenties, maybe called at 17 and survived the Cruciamentum. (If she's good enough that Spike compares her to Buffy, she should be able to pass.) And if she was that much like Buffy, she could've survived longer than most Slayers do.
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Well, we don't know how it works at all, or if it's even the Powers that make the selection (my impression is it's not, if only because the Powers weren't a part of BtVS, and thus it's unlikely they were meant to play a role in Slayer mythology). But I would guess the Council has the best understanding of the parameters, just by virtue of studying the patterns of Slayers called.
Like I said, bad with guessing ages, but I could see Nikki being early twenties, maybe called at 17 and survived the Cruciamentum. (If she's good enough that Spike compares her to Buffy, she should be able to pass.) And if she was that much like Buffy, she could've survived longer than most Slayers do.