New analysis by the Press Association found that a decisive result on June 23 could depend on whether people aged 18-24 stay away from the polls in similar numbers to last year, when only four in 10 voted - compared with over three-quarters of people aged 55 and above.
The PA’s projection of the outcome of the referendum, based on the latest opinion polls and using demographic data modelled on the 2015 election, shows Remain and Leave currently tied on 50.0%.
But if turnout on June 23 is just two percentage points higher among voters aged 55 and over, the projection changes to a narrow victory for Leave.
A one-point rise in turnout among 18-24 year-olds tilts the result narrowly towards Remain.
The findings come as time is running out for people to register to vote in the referendum, with the deadline 11.59pm on Tuesday.
Turnout among 18-24 year-olds at the 2015 general election was just 43%, compared with 77% of 55-64 year-olds and 78% of people aged 65 and over.
Almost every poll published since the campaign began has suggested that a clear majority of older voters favour leaving the EU, while as many as four in five younger voters back the UK to remain.
The PA’s investigation also found that:
• If turnout among 18-24 and 25-34 year-olds drops by just three percentage points compared with the general election, and all other figures remain the same, the projection changes to Leave 50.2%, Remain 49.8%.
• By contrast, if turnout among those two age groups rises by two percentage points, and all other figures are unchanged, Remain would finish on 50.2% and Leave on 49.8%.
• If turnout rises across all ages, but by a greater amount among 18-44 year olds (10 points) than among people 45 and over (five points), the projection widens to Remain 50.5%, Leave 49.5%. If those figures are reversed, Leave has a narrow win of 50.1% to Remain’s 49.9%.
A survey published last week by the Electoral Reform Society (ERS) found that while 47% of 18-24 year-olds said they would definitely vote in the referendum, the figure for those aged 65 and older was 80%.
Darren Hughes, deputy chief executive of the ERS, said: “Around four million 18-24 year olds are unregistered. We need extra efforts to encourage them to sign up in colleges, universities and workplaces across the country.
“This referendum shouldn’t be decided by one generation on behalf of another - this is a vital national conversation that needs to involve everyone, not just older voters.
“Let’s call time on the EU referendum generation gap to make sure this really is a truly national conversation.”
People can apply to register to vote online at gov.uk/register-to-vote.
• The PA’s analysis is based on the findings of every opinion poll published over the past month by BMG, ComRes, ICM, Ipsos-Mori, Opinium, Survation and YouGov (20 in total). It uses estimates of the size of the electorate and the population from the Electoral Commission and the Office for National Statistics.