Only 36% of 18-24 years old voted in the EU Referendum.
From The Independent UK:
"The 51.9 per cent to 48.1 per cent was so close – if the rest of the young had voted, the outcome could have been very different. The gap between Remain and Leave was 1,269,501 votes. The estimated population of 20 – 24-year-olds in 2015 was 3,806,471 – only 492,306 applied to register to vote in the months running up to the election. Many young voters would still have been on the electoral register from the General Election and local elections – this figure is to show that of the 2 million people who panic registered in the last weeks, only around one-quarter were young voters."