Maybe they don't vote because they think the system is broken. No one has informed them that our institutions are designed to clang against each other, to keep one from running away with the whole country. Perhaps they hold the widely held and perfectly understandable misapprehension that the purpose of American politics to sweep the bad guys out and bring the good guys in, rather than transfer authority from one set of imperfect leaders to another. Add to that the relentless cynicism and calumnies they are fed by 1960s radicals in college, and elections must seem like quite a turnoff to them indeed.
But they are undeniably smart folks, and they will live and learn. They will see that when Democrats have things all their own way, they run Detroit and California into the ditch. They will see that when Republicans have things all their own way, they run Kansas into the ditch. The Millennials will draw the appropriate conclusions, once they are finished with the usual adolescent outrage at the world's tardiness in remaking itself to their liking. The country will be in good hands.