tl;dr: young people are stupid. and the right-wing are stupid. Stupid, left me introduce you to Stupid.
Which is a stupid position, because
people are stupid, if you're going to vomit stupidity claims all over the place. Young people aren't much more stupid when it comes to political involvement than old ones, if at all, and just as impacted by the various bits of stupidity that emerge from it as anyone. They have as much right to a say as every senile fucker in my town, if stupidity is going to be the what you evaluate whether they get political franchise or not with.
... also, quite frankly, if it led to the GOP pandering to issues primarily screwing the youth, even short term ones, instead of buggering everyone to pander to the rich and/or old, mission bloody accomplished. Sub-mission, anyway.
Are 16-18 year olds still pretty much required to go to school in America? Or have to explicitly 'drop out' instead of just choosing to not sign up for the next two years. High School is until 18 in America, right? Basically, are those two years 'opt-in', or are they 'opt-out'? 
It's opt out, though not necessarily until you're 18 per se. It's grade based, not age based, so you get variance.
That said, if you're going by that sort of evaluation, I'd still hand it out around age 16; that's around the point dropping out actually becomes legally feasible without substantial parental support, iirc, i.e. it's when we think it's okay for people to go screw themselves and die in a ditch. Might as well let 'em vote if they want to vote.
... though yeah, at least part of the alcohol thing is driving (and guns, for that matter) is a much bigger issue in the US than in the UK. Drunk teens get enough people killed or hospitalised as is to make it even easier for 'em to get at the stuff. Not that underaged drinking laws are particularly enforced, but eh.
Some online articles tried out a questionnaire you have to fill out showing that you read the actual article. Quality of discussion shoots up amazingly high.
Maybe we should allow eveyone to vote on any issue, however you also have to fill out a multiple-choice questionnaire about what your voting on for your vote to count. Then, sure, misinformers on either side could use misinformation to get their supporters out on election day, but it would backfire.
Means testing the vote in any way, shape, or form, is stupidity of the highest goddamn order, unfortunately.
Maybe if the questionnaire didn't have a failure state and it didn't matter what was answered, I'unno. It's still a proposition riddled with problems from who designs to questionnaire, to when it's filled out, to the costs involved and gods know what else. There's plenty of reasons we don't do that shit anymore.
