They polled them. I can make out under your line that "Under ?5" (presumably "55") is 53-45 in favor of Sanders. But the smaller age breakdowns were too small.
Is this a deliberate avoidance of polling younger voters in order to boost Clinton? Or did they try polling evenly but their methodology is outdated and skewed older? Or are they getting an accurate sample of voters and the boomers are just vastly outnumbering everyone else? I don't think the answer is clear.
But I feel like drawing your circle in a way that obscures the "Under ?5" demographic which did favor Sanders and then saying that they didn't poll the demographic that favors Sanders comes off as shady. Like the pollsters, it's not clear whether it's deliberately misleading or a simple accident.