I've come around to the idea recently, despite always having concerns about it.
You can go on all you want about young voters not understanding the issues or being influenced by sensationalism, but you could say exactly the same about a large proportion of adult voters, so I don't really think that's a valid excuse to deny it to under 16s. Young people see sensationalised rubbish on social media, but middle aged and older people read sensationalised rubbish in the newspapers. See the Daily Mail and the Sun for examples of that.
Plus it would give an opportunity for more young people to get involved in politics. Secondary schools would have several entire year groups eligible to vote, instead of just one or two of the oldest sixth formers. That brings opportunity to discuss stuff in school. People say young people don't take it seriously, but maybe they would if they were allowed to vote, as we saw with the Scottish independence referendum? Those who don't take it seriously are less likely to vote anyway.
We have the answer...