I doubt the youth vote will deviate significantly from historical patterns. In 2012, it was already pretty low. People like to say Obama won in part by turning out the youth, but in reality youth turnout was actually lower than the average in the modern era (1972 - now). The reality is that Millennials are growing up and voting more as they age, and Millennials are heavily Democratic. That is what he owes part of his success to.
If I had to guess, youth turnout will stay at 2012 levels, give or take a little bit.
http://civicyouth.org/quick-facts/youth-voting/
Poll
