Andrew Sullivan is in one sense correct: traditionally, the youngest cohort of voters dont' show up at the polls. But the past two election cycles seem to be the start of a new trend: in '04, the percentage of 18 to 24 year olds rose by, I think I'm remembering this correctly, 11 percent, far exceeding the turnout increases among other age groups. True -- they still vote at lower average rates, but it's not possible to dismiss their influence in close elections anymore.
A worrisome sign for Republicans: as John Judis and Ruy Teixiera note in the latest American Prospect, these millennials are much more Democratically-inclined than Generation X was: they cite a Pew survey showing that 60% of 18-29-year olds voted for the Democratic candidate in '06.
To Andrew's point: in primaries, the Millennials matter more in some states rather than others. Iowa is the oldest state in the union, demographically; young Republicans typically don't caucus in numbers proportionate to their strength. New Hampshire is different, although young independents are probably inclined to vote for Democrats in 2008.

