Nothing specific to Nikki. Just repeated assertions by almost everyone on the show that Slayers don't live more than a few years past being called (or past their 21st birthday, etc), and that Buffy is an extraordinary exception in her length of survival.
...apart from the fact that we've never seen anyone called at an older age, and if Nikki was called in her teens, she clearly has survived for longer than usual.
Eh, we never see a Slayer get vamped but we recognize that may have happened at some point. Unless we see something on screen that tells us Slayers can't be called at a later age, I don't see why it should be concluded that it's impossible.
The old (pre-S8) comics do mention a Japanese Slayer who was turned a couple of hundred years ago; the Council hushed it up as a big scandal and struck her name from the records. Lots of angst because Buffy met her and Giles refused to explain who she was, but it turned out that Spike knew her from the old days (she was turned by the Master, in fact).
So according to a deuterocanonical source at least, turning Slayers is possible, but very, very rare.
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I'd imagine that there have been quite a few Slayers, over the centuries, who have been Slayers for more than ten years. It's just not the norm.
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So according to a deuterocanonical source at least, turning Slayers is possible, but very, very rare.