Mitt Romney did worse across pretty much all age cohorts compared to President Bush in 2004.
Bush only won 43% of the 18-24 year-old cohort. There wasn't much of an outcry about Bush's young voter problem because he won. Two elections later, Romney won about 40% of the now 26-32 year-olds. This 3pt drop was entirely consistent with a national Republican drop of 3.5pt from Bush's 50.7% to Romney's 47.2%.
Other age groups are consistent with that effect. Bush won 53% of the then 30-44 year-old vote. This time, Romney nabbed 50% of the now 40-49 year-old cohort, which again matches the Republican drop of 3pt nationally. Same with the then 40-49 year-old vote, where Bush grabbed 54% and Romney walked away with 52% of the now 50-64 year-old cohort.
Very well, you might say, but what about the addition of voters who turned 18 since 2004? Surely, they have tilted the electorate. Yet, the now 18-25 year-olds mostly took over for the 75-plus year-olds in 2004, who were very Democratic. That's why age cohorts who cast a ballot in 2004 and 2012 voted the same relative to national vote, even as more millennials have turned 18.
Of course, a steady parade of Democratic millennials could make hell for the Republican party. They will be replacing the 60-74 year-olds of 2004 and now 68-82 year-olds, who have been 6-8pt more Republican than the nation as whole.
The good news for Republicans is that the new 13-18 year-olds don't seem to be like today's 18-32 year-old voters. As I noted last week, the men and women college freshmen of 2012 were 4-5pt less liberal than those of 2008, which brings them closer to middle-of-the-road freshmen of the beginning of the Carter and Clinton administrations.