why would I trust the rest of societies kids to be more thoughtful about it than I was?
This is fair enough, but before women had the vote it was assumed that they couldn't vote sensibly, same with those without property. They were definitely wrong about the women, and allowing those who didn't own anything to vote changed British politics immensely, on balance probably for the better.
I rather assume that many 16-17 year olds wouldn't vote, and that their voting might be unpredictable, a bit whimsical and perhaps not terribly well informed about the issues that matter to me. That doesn't necessarily mean that it owuldn't make a positive difference to British politics and government to have them voting.
Also, a huge number of adults have the vote who are stupider and vastly more ignorant than I am, but I reckon I have to let them vote. Even when they may have considerably more to gain at my expense from (for example) a huge hike in the top rate of income tax than I can ever hope to gain from my voting preferences translating into government policy. 16 year olds aren't (needn't be) stupid, and actually engaging with them and getting them voting might improve the quality of their thinking. I don't think there's a huge difference between 16 and 18 for these purposes. 12 would be a different matter I guess.