True, though I'm not so sure on the age restriction there. Nikki appeared older than 18, and it's unlikely she survived more than a few years as a Slayer. I don't know that the Powers work like that. I'd imagine that an older Slayer may be rare, but possible. The Council would likely try to get rid of them some other way, perhaps, since an older Slayer would be less likely to obey them (as...does actually happen with Faith).