Basically, the statistics bear out that the "Berniebro" doesn't really exist any more than Hillary voters who only support her because she's a woman do. They're certainly out there as anomalous individuals, but they're not really even a "thing", let alone a force or something worth talking about.
They're convenient memes for the media and partisans to latch onto which topically and anecdotally people affirm knowing one or more such people...but the data bears out the bullshit of editorial pieces like Amanda Marcotte's at Salon and this one by Robinson Meyer at The Atlantic. It's not about commentary or reflection of reality...it's just about grinding their ideological axe.
Although Marcotte insists that she couldnt find good polls on gender amongst supporters, much less age, data for both are readily available on the Internet.
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Of course, my points here all assume that election takes are actually about the election. Its clear in the case of pundits like Marcotte that election analysis is just a continuation of their normal personal political blogging. It is a new frame within which they hash out their same themes and grind their same axes, not a genuine effort to figure out whats going on out there in the vast world of the Democratic electorate. Its not just her who does this. I would say the majority of election punditry is like this, including on the Right.
https://www.jacobinmag.com/2015/10/clinton-sanders-democratic-presidential-primary-caucuses/