General election 2017: How reliable are the polls? [View all]

"There was a particular problem with young voters. The ones who agreed to answer pollsters' questions tended to be more interested in politics and more left-wing than young people generally.

"That led to bad estimates of how many young people would actually vote. We've known for a long time that young people are less likely to vote than older people. But what the polls failed to pick up was the size of the turnout gap between young and old. That led them to overestimate Labour's share of the vote."
...
"ComRes and ICM now estimate how likely somebody is to vote based on their age and class background. Broadly speaking, young working-class voters are assumed to be much less likely to vote than older middle-class voters even if they say that they will do so. That tends to suppress the estimate of Labour's vote share.

"YouGov and Ipsos MORI have also made changes to how they estimate turnout. They now take into account whether respondents have voted in the past or whether they usually vote. However, these changes have a much smaller impact on their topline voting intention numbers than the ComRes/ICM approach. That means they suggest a closer contest."

http://www.bbc.co.uk/news/election-2017-40123231

Interesting, since the difference in poll numbers has been considerable. I had been trying to work out how this ICM poll went from a Con/Lab/don't know figure of 35/29/16 to a "likely vote" Con/Lab split of 48/33. Even with their "how likely are you to vote" figures, it made no sense, but if they were assuming that Labour would lose young voters even if they said they were likely to vote, that could explain it.