The Monmouth poll is here.
In the last three presidential elections voters under 45 years of age actually turned out to vote at an average rate of 52% compared with 70% for voters 45 and older. Almost half the sample in a recent Monmouth presidential poll is populated by that younger demographic, which means that the part of the sample which is more likely to represent Democrat preference could be overstating their likely election turnout for Hillary in 2016 by almost 50%.
A notable feature of the last three presidential elections has been the enormous uptick in voter turnout by young people aged 18 to 29, most of whom have been for Bernie Sanders this cycle, not Hillary Clinton. Turnout has averaged 45% for this age group since 2004 compared with 37% prior, and given the lack of enthusiasm for Hillary the turnout for this demographic may well revert to the status quo ante.
Excepting the election in 1992 when the 18-29s turned out for Bill Clinton at almost 43%, in 1988, 1996 and 2000 they yawned Meh! turning out at an average of 34% compared to 66% for their elders aged 60+.