Not just the young; the industrial/ northern working class as well. Observe that it wasn't just a depressed turnout for the Tories after an abysmal campaign: the youth turnout may have still been unimpressive, but Labour's turnout was significantly higher nationally, beating Miliband, Brown, and Cameron. Labour's 2017 voteshare would have been enough to win any previous election since 2001.2017 was an unexpectedly good peformance for three reasons. Firstly, the fact that Jeremy Corbyn had been hiding under a rock as leader as he was thoroughly traduced, and this finally allowed him to get some camera time and turn out to be not as bad as he'd been portrayed. Secondly, the worst Conservative general election campaign since 1945. Thirdly, that the election was in part a continuation of the Brexit disagreement, where Remainers heavily threw their lot in with Labour (as Brexiters jumped ship to the Tories).
In the excitement of Corbynmania, the turnout for under-35s went from around 48-52%. Given the proportion of the population they represent, that equates to about an extra 1% to Labour's vote share. Labour needs turnout in this age range to be vastly higher, and there is no guarantee it ever will be: youth turnout has pretty much always been substantially lower than the national average.
The Labour base will not win an election. Never has, never will. This is not the USA, where two monolithic parties split the country pretty much 50:50. Labour and Tories alike rely on bases which are a much smaller percentage of the electorate with a much higher percentage of voters willing to switch party allegiance.
Some of that is due to courting new sectors and new voters. But its more explicable through turnout.