But look closer, and you'll actually notice that the anti-millennial grievance has switched between two lines of criticism, each equally baseless in their own way.
Here, in argument (1), millennial support for Clinton is inadequate compared to other age groups. Specifically, olds like to point to the comparatively high youth support for third-party candidates in order to suggest that support for Clinton comparatively low. This of course is demonstrably incorrect on multiple grounds. Voters under 30 give Clinton her second highest margin of all age groups, and millennials (when we bracket generations correctly) give her a margin greater than every other age group combined. And while millennials may vote for third parties in high numbers, most of that attrition is coming from Trump's camp, not Clinton's. For instance, among supporters of Gary Johnson, every other age group has a significantly higher attrition rate from Clinton's camp than millennials do.
Thus, argument 1 fails completely: millennials are voting in higher numbers for Clinton than anyone else, and defecting from her far less.
That's why, particularly in the past week or so, we've seen a slightly different complaint:
In argument 2, millennial support for Clinton is inadequate compared to their support for Obama. The premise here is that we should expect age groups to give Democrats a certain level of support based on the last election, and that their failure can be measured by how much that support declines.
It should probably be enough to say that pointing to the last election really just defers the question: instead of asking "are millennials underperforming," we're now asking "did millennials overperform last time?" But instead of leaning too hard on that, I'd like to make a distinct point:
18-30 year old voters
Clearly, if failure is simply measured by decline, then young African Americans are failing liberals more than anyone, followed by Asian Americans and then Latino/as. Any shift among support among young white voters, meanwhile, is so small that it's barely outside the margin of error (as is any current difference between young whites and older whites). So you cannot, that is to say, talk about the decline in youth support for Democrats without laying almost exclusive blame on young voters of color. Somehow, I doubt that Clinton's (largely white) media surrogates will have the audacity to rally behind that complaint.
What if Clinton does not actually have a millennial problem
Often when a grievance has to rely on multiple, shifting lines of critique, we can take this as a good indication that the arguments are just being backfilled to support the complaint. This is particularly true when the criticism turns out to be unusually baseless and flimsy; when this happens, it often makes sense to start looking for motivation. One egregiously incorrect data mistake is understandable - two are suspicious, and beg for an explanation.
Here, I think the explanation is pretty simple. Clinton's lead over Trump has tightened over the past month, to the point of occasionally disappearing altogether. This is making Clinton supporters nervous, and naturally they are looking for someone to blame. And because olds are significantly overrepresented on the editorial boards and in the opinion columns of elite media, this anxiety will predictably express itself as an inclination towards blaming young people. The persistence of this grievance says almost nothing about the actual numbers - past or present - but it says a lot about the ageism in our media.