David Cameron and Alex Salmond are to meet on Monday to finalise arrangements for the planned referendum on Scottish independence. But three salient details have already emerged. The first is that the vote will be held in 2014, so it will be ahead of, and quite separate from, the next general election. That is the right decision, and a point to Mr Salmond.

The second is that there will be just two options: independence or not, with nothing, such as so-called Devolution Max, in between. That is right, too, and a point for Mr Cameron. This has to be a straight vote on independence, no more and no less, designed to produce an unambiguous answer.

It is hard to be as enthusiastic about the third detail – the extension of the vote to 16- and 17-year-olds. Our qualms do not relate primarily to the readiness or otherwise of 16- and 17-year-olds to vote. That is a separate question, and there may well be a discussion to be had about whether young people whom the State deems old enough to marry and join the armed services (but not to fight, buy cigarettes or a drink, or, after 2013, leave school) should also have the right to vote.

Our misgivings relate rather to the political considerations that lie behind the decision. The argument advanced by the SNP is that the franchise should be extended for this one vote, because it is younger Scots whose future will be most affected. But polling also shows younger Scots to be disproportionately in favour of independence, so the decision could well boost the Yes vote – as Mr Salmond well knows.

A one-off reduction in the voting age for political reasons is highly undesirable in itself. But such a change would inevitably set a precedent that would be extremely hard either to restrict to Scotland or to reverse. If the UK voting age is to be lowered, this should be done only after a thorough debate about the pros and cons, and a parliamentary vote. It should not be introduced by the back door as part of a sectional political deal.