Before the second world war the voting age in most countries was 21 but in the second half of the twentieth century the age of 18 gradually became very common. For example, the USA passed the 26th amendment to its constitution in 1971 largely because it was felt that to be conscripted at age 18 and not allowed to vote was anomalous. There are a few countries where the voting age is 16 (such as Austria) and a very few where it is higher than 18 (such as Kuwait and Bahrain). In many countries there have been movements to bring the age down to 16 (for example, the UK, Australia, and New Zealand) but these attempts were abandoned (although, for the 2014 Scottish Independence referendum, the voting age was 16).
The reason that 18 has emerged as the most common voting age seems to be that 18 is seen as the age when children become adults. That this transition should occur instantly on an eighteenth birthday is obviously a fiction and therefore it is legitimate to ask whether we have got the age right, and to ask whether adulthood is the criterion we should be using.
The assumption that adulthood is a prerequisite to vote is very ingrained. Surely, it is said, a certain maturity is essential before a considered vote can be cast. And yet the right candidate to vote for and the right set of policies to support are generally not decisions that are arrived at by the tools of knowledge and logic; because, if they were, there would be no dispute at the ballot box. Once one accepts that opinions are driven by prejudice and emotion who is to say that a 16 year old is not as qualified as an 18 year old? Let's be honest and admit that most people vote out of self interest even if they are able to rationalise their decisions.
So why should the voting age not be 16? But isn't that as arbitrary as 18? If you are thinking along those lines consider the very bold proposal by Cambridge Professor David Runciman. He suggests the voting age should be lowered to 6! He has admitted that his proposal is made with a certain tongue in cheek but is ready to defend it anyway. For example, what of the objection that children will just vote as their parents tell them? Why should they? Women did not vote the way their husbands said they should when they got the franchise?
For me, the most compelling reason for taking Runciman seriously is that our electorate is already top heavy with old people. The most compelling issues of our age - climate change, whether Scotland should have independence, how should institutionalised racialism be addressed in the USA and other countries - are long term issues. Why should old people have any say in these? Leave it to the citizens who have the most skin in the game. After all if, like me, you are appalled at how the majority of the UK voters voted in the EU referendum, consider what might have happened if the franchise had consisted also of 16 and 17 year olds (a proposal that was seriously floated in 2015). Don't you think their youthful good sense would have prevailed over the grey vote which, on the day, carried the Leave side over the line?
