Why has grey hair evolved?
Presumably this means that at least some grey haired humans have
  noticeable reproductive advantage, or maybe they had it in the recent
  past.


No it doesn't.  Natural selection is not that strong, it doesn't optimize every single possible physical trait towards maximum reproducing.

And as others have mentioned, having lots of grey hair usually happens after reproduction is over.  Historically, lots of women did a lot of reproducing before they had any grey hair.