This is already the first election in which millenials and gen-Xers (terrible name, btw) outnumber them as far as eligible voters. The problem is eligibility isn't enough - old people vote far, far more reliably than young people, so the (often idiotic) preferences of their demographic still win out.

Automatic national voter registration when turning 18 would be a step in the right direction, but I'm not sure it would convince people to actually go to the polls. In certain states just registering to vote in the first place can be a real hassle, and having to do it every time you move punishes the more mobile generations.