Earlier quoted context omitted.
Voter turnout was at record levels. It's not obvious that greater turnout of "millennials" specifically was likely to happen. Typically it's difficult to ensure that one kind of voter turns out without also increasing the rate of turnout for other kinds of voters as well.
But voter turnout for younger people (18 to 24) was less than 40%. If they'd had 75% turnout they may have swung the election. In mid 2013 there were about 5.8m people between 18 and 24.
Instead, maybe they actually were apathic, and their votes would be roughly 50-50 - so Leave could still win.