However, annual state-by-state population estimates from the US census bureau suggest this demographic makes up roughly a quarter of the electorate - this can vary on a state-by-state basis, of course.
In addition, exit polls suggested that this demographic were in favour of Clinton over Trump by 55 per cent to 33, (12 per cent voted other).
A point of interest has been what millennials could do to the election results - this survey by surveymonkey has gathered particular interest, despite the unreliability of online polls.
It will be very hard to answer this question until the Federal Election Commission releases the full election results dataset, until then we're all stabbing in the dark a little with exit polls. We thought we'd add our attempt.
So what would happen if millennials were the only ones to have voted in the election?
Clinton wins with 420 electoral votes to Trump's 118.