@Grammarnut,
Yes, you're right. But I'm not saying I'm in favour of enfranchising 16 year olds, just that the idea isn't barmy. I can see it happening, but perhaps not in the immediate future. People enter adulthood at different ages for different purposes. A 16 year old can ride a moped or dirt bike on the road. You are criminally responsible at 10 in England and Wales, 12 in Scotland. But I'd bet a fiver that if the age was reduced, everyone would be amazed at how boring and conservative the 16-18 years are (at least, the ones who bother to get up).
Any argument which demands competence in voters is inherently undemocratic, as it gives the government too much power. "Competence" has never been used to grant the vote, only to deny it to disenfranchised groups. I've been voting for nearly 50 years, and no one has ever asked if I was a fit person to do so or if my judgement was sound and well informed. And neither should they! Can't have the government deciding who can vote for it and who can't.
And, by the way, that's why the vote shouldn't be taken away from prisoners routinely. Disenfranchising groups should not be in the routine power of the government. If a prisoner is to be stripped of their voting right, I want to hear a judge say it, each time, with reasons. Otherwise the vote stops being a right and degenerates into being a licence, held at the government's whim.