Again, the data say otherwise. Unless the young were suddenly spurred into voting for Corbyn on the same day as the Conservative manifesto for some entirely unrelated reason.Except they were the only pollster whose polls showed a Hung Parliament...
I know your right-wing mindset can't comprehend it, but the youth vote and former Labour voting Ukipers voted for Corbyn's manifesto and this changed the face of the election.
Disagree all you want but it is staring you in the face.
The far more likely scenario is that younger voters would have answered Labour when polled all along, they're certainly all been claiming so on Facebook. The swing started on the day the Conservatives published their manifesto.
As for Survation, whilst they got the seats reasonably close, they were wrong on the vote share - their final survey only had half of the Conservative lead on share of the vote. And we have to use vote share when tracking the timeline as that's the only data available over time.