Check out what 1967Wedway says (also so well) in response to James Boyle's opinion (and apart from my other point):
Immaturity, incompetence, and apathy are very real issues among voters over 18, but accusations of these things aren't used to justify withholding their right to vote. So why is it that people under 18 are accused of these things to withhold their right to vote? Every citizen has a stake in government, so it is crucial that every individual be entitled to a say in the way it is run. I'll never understand why so many people are content with the government deciding who may vote for them.Here is a 2008 commercial on behalf of lowering the voting age to 16 from the National Youth Rights Association.
Politicians make decisions every day that critically affect young people's lives and futures, and yet, young people have no say about it. They may work and pay taxes, volunteer labor in their communities, live on their own, have children themselves, struggle to obtain an education or find affordable health care or jobs, worry about paying for college, medical bills, housing, transportation, food; they have a stake in what happens to social security or their tax dollars. They can't vote, but the government is already recruiting them in public schools to take on staggering student debt, or go fight in their wars that never seem to stop needing more cannon fodder from their ranks. Definitely lower the voting age.