So Gary Illyes from Google was at this event, which I was not at :), and he said at the event that the mobile first index might not launch in 2017. He said he hopes it does launch but he is not sure.
Let me share some of the tweets from the event, because I wasn't there, so it is hard to just go by one tweet:
This was disputed:
But then Gary himself said it doesn't look likely that it will launch by the end of this year:
So it looks like the Google mobile first index likely won't launch in 2017, maybe it will in 2018?
Forum discussion at Twitter.