I've tossed around this notion twice in passing recently, but let me take a minute to lay it out with a little (very little) more substance.
Why is the voting age 18?
The voting age was lowered from 21 to 18 during the Vietnam War, when individuals too young to vote were being drafted. Although several states had already lowered the voting age, the highest minimum voting age was set nationally by the 26th Amendment, reading "The right of citizens of the United States, who are eighteen years of age or older, to vote shall not be denied or abridged by the United States or by any State on account of age."
Note that it doesn't say no one younger than 18 shouldn't be allowed to vote -- just that no one older than 18 can be denied the right to vote.
Why lower the voting age now?
Ideologically, simply because I think 17 is old enough. The child-adult boundary is increasingly drawn at 16: the federal chld labor laws apply to those 16 and under, the age of sexual consent in most states is 16 or 17, and the military's minimum enlistment age is 17 (though that does require parental consent, and the minimum combat age is 18). In recent years the army has been aggresively recruiting in high schools -- in some cases, VERY aggressively. In fact, No Child Left Behind said that high schools were unable to keep military recruiters away from students without forfeiting federal funds. I think that if you can drive, work, have sex, and join the army, you are grown-up enough to vote.
Politically, because it is a win for Democrats across the board. What's more, we are approaching perfect storm conditions for this kind of movement. There are about 4 million 17-year-olds in the US, who are overwhelmingly Democratic. Young people are being drawn into politics like never before through the Obama campaign. We are heading into an election where a major defining contrast is between youth and age. We have a Republican party wrapping itself in "support the troops," who will find themselves taking a stand against the rights of the youngest military personnel. At the state level, Democratic governors are in the majority for the first time since 1992, and half of our Democratic governors control states that Bush won in 2004. Even if we can't reasonably lower the voting age before November -- and, frankly, I don't think we can -- a campaign to do so at the national and at the state level would further energize the youth movement, would underscore the differences between the parties, would put Republicans in an uncomfortable position, and would lay the groundwork for real change in coming years.
How would we do it?
I can imagine two ways. First, Congress could either pass a Constitutional amendment or a law lowering the voting age to 17. The Supreme Court has ruled that the federal government can't set the voting age for state elections (that's why the current voting age is set by a Constitutional amendment), so if Congress passes a law it would need to be written so that 17-year-olds could vote in federal elections but not on state ballots, a recipe for confusion at the polls. One way around this might be to treat 17-year-olds nationally similar to the way Americans abroad are treated, who can vote in presidential elections but not on state matters. I have no idea how this would work.
The second approach is a series of state-level laws or ballot initiatives. This approach has the advantage of keeping the movement local, building local activism, getting young people learning the political system from the bottom-up. And, as noted above, Democrats control the governorships of 14 red states, and swing states like Ohio and Pennsylvania. Lowering the voting age in a few swing states alone could have as much impact on the election as a national measure might.
Lowering the voting age to 17 would add a few million Democrats to the rolls, capitalize on the youth-powered movement that Obama has begun, and put Republicans in the uncomfortable position of arguing that the youngest enlisted military personnel should not be allowed to vote.