There are approx 6.5M 18-24 year olds in the UK. This is nearly 10% of the 65M population and 13% of eligible voters.
In the recent Brexit vote, 82% of those 18-24 expected Remain to win in the lead up to the election, but only 36% of them registered (a subsection of those eligible) showed up to vote.
More generally, turnout in this age bracket has fallen from over 60% in the early 1990s to an average of 40% and is estimated around 36% for the recent Brexit vote. According to the British Election Study, the proportion of registered 18 to 24-year-olds turning out to vote has consistently been below every other age group since the 1970s. Eurobarometer suggested that young British adults are among the least engaged with the political process when comparing their peers across Europe. Young people do not only vote less, but research by the Electoral Commission suggests they are less likely to be on the electoral register than other age groups.