Wait, is Trump really leading the youth vote in this poll?
Younger people are also a bit difficult to contact for polls, or if you're phonebanking, or canvassing. Especially how many would have the 'wrong' phone number on record or a household number or something not a cell phone. If you can get them to pick up.
So, out of 814 people polled, how many were in the "youth vote" category, 100-150? So, it's not going to be representative.
Polls usually underestimate Dems with young people and various minority groups, and probably overestimate them with white people and old white people