A tremendous amount of assumption here about how young people behave and how they would vote.
Thank you Tuliptree for the link to the Lords library paper:
David Runciman, honorary professor of politics at the University of Cambridge, argues that “no-one should assume they know how young people are going to vote—the point of any democracy worthy of the name is that it should be unpredictable.
According to the latest Office for National Statistics (ONS) figures, there are around 1.6 million 16 and 17 year-olds in the UK. While this sounds like a big number, it is equivalent to less than 3% of the UK’s entire over 16 population, so it would represent only a small increase to the overall size of the electorate (though not everyone included in the ONS population estimates is entitled to vote—for instance those who do not meet nationality requirements).
In Westminster’s first-past-the-post electoral system, what would matter is not just the total number of potential new voters created by lowering the voting age, but also their voting pattern across individual constituencies. ONS analysis of the 2017 general election results found there were 88 constituencies (around 14% of seats) where the number of 16- and 17-year-olds was greater than the winning margin. House of Lords Library analysis based on the same methodology found this rose to 120 constituencies for the 2024 general election (around 18% of seats). Writing about the 2017 election, the ONS noted that if 16- and 17-year-olds had been able to vote in such seats, it would not necessarily have changed the outcome—it would depend on their rate of turnout and which candidates they decided to vote for. Dr Christine Huebner of the University of Sheffield and Dr Jan Eichhorn of the University of Edinburgh—academics whose work examines the impacts of lowering the voting age—observe that “no voter group participates at 100%, and young people do not vote homogenously, so the actual number of constituencies with potential changes to election outcomes would be much lower.