That demographic could guarantee Obama easily . . . the problem is that they don't show up to vote. They flake out, they don't register, the aren't registered in their college town, etc. But the senior citizens that have nothing better to do with their lives ALWAYS vote.
All of which is offset by demographics. You realize that the 18-29 age bracket is bigger than it was four years ago right? I don't think you realize just how large Generation Y/Millennial is. Especially compared to the relatively small Gen X, which used to be the 18-29 bracket a very short time ago.
Right now the 18-29 bracket is all Gen Y/Millennial and still getting larger. That's four more years of "Silents" dying off and four more years of "Millennials" coming of age to vote, who are also double the size of the "Silents" Generation.