Let's talk likely voter screens. We will see more polls similar to the Iowa one

First, my general disclaimer: I am a Clinton supporter, but I am also a statical analyst. When talking polls I try to be factual on why they're set up the way they are and where methodologies may fall apart.

Anyway, the Monmouth poll is a likely voter poll and not just a registered voter poll. Most polling that is occurring right now is for registered voters, which will begin to switch to likely voters as the primaries draw closer.

Registered voter polls are pretty simple affairs--if you're registered with a party then your answer is tallied.

Likely voter polls, on the other hand, attempt to answer the question of who will actually come out to vote when the election or caucus arrives. For proper statistical consideration you can't simply ask the person and take their word that they will come out, so instead the vast majority of polling agencies use past history. Specifically, has that person voted in the past. Then they ask are you planning on voting in the next primary.

Unfortunately, the last Democratic primary was 8 years ago, which means that only millennials who are 26 years or older now will trigger the past history requirement. All of the college-aged kids who have registered for the first time are not considered likely voters and won't be by most agencies. And it should be noted that specific age group is notoriously unlikely to vote in a primary.

So you can see why likely voter screens tend to older voters.

Monmouth appears to have a very aggressive weighting mechanism attached to their current poll, and I do believe it is underrepresenting turnout for the under 50 population (In 2008, 27 and under were 22% of the caucus demographics). But that does not mean this is a deliberate attempt to prop up Hillary.

The fact is, likely voter screens, as they roll out, will discount millennial responses. While you might not see something as outrageous as Clinton being +41 in Iowa, Sanders will lose points when pollsters switch from registered voters. It is one of the primary risks of counting heavily on the youth vote in polpoint

Edit: Case in point, the Loras poll just now being reported is also a likely voter poll.