Polls are conducted by phone.
Surprisingly few people under 35 have landlines. The get their internet from cable, and their phones are wireless. This means that the youth vote has no voice in most polls.
The polling company that wants to be most accurate will soon have to send text messages to wireless subscribers. "Be Polled! Return this call to register your opinion. Press 1 for Democrat, 2 for Republican, 3 for No Opinion. If you press a 1 or a 2, you will need to enter a candidate number...."
I suspect that the polls would change percentages in interesting ways if methods were updated.