Poor voter turnout amongst the youth in the United States is caused by several factors:
1) Voting does not happen on a holiday. If you can't get off work and if you can't get someone to watch your kids, you aren't going to vote. Early and absentee voting is possible in 34 of 50 states, but a surprising percentage of the population can't get their shit together.
2) Cynicism and discouragement reduce turnout. The electoral college system makes it so that there's a real sense one person's vote doesn't matter. That candidates will visit every.single.fucking.mailstop in swing states while utterly bypassing, oh, California (where more than 10% of the population of the United States lives) doesn't help - especially when 325 voters in Florida held more sway than California in 2000 and that Clinton won the popular vote by more than the population of both Dakotas and Montana combined but still lost the election. That these voters can affect local politics hardly matters because local politics have much less impact than national politics.
3) Old people have nothing better to do. It's easy to have civic engagement when it's the only fucking thing you have going on.
A comprehensive voter rights bill would go a long goddamn way but in the current climate, Kris Kobach is a potential cabinet secretary so it's going to be a while before we can get our heads out of our asses.