I think people miss the point a little bit when they look at Rubio's response to the question about the age of the earth in the context of Iowa. The real state to watch for with Rubio is New Hampshire. Right now Huckabee is leading the polls Iowa. Let that sink in a bit, Huckabee. Rubio's better path to the nomination, should he choose to accept it, would be to follow the McCain strategy and do well in New Hampshire, where a more moderate, independent Republican can do well, and then dominate the Republican primaries in the large blue states. For two cycles now Iowa has nominated an unelectable fundamentalist and has diminished its importance in the process. Rubio is better off skipping it and scoring a decisive win in New Hampshire, where an ambiguous answer on creationism is more useful than the definitive response that would play well in Iowa.
Personally, when I heard Rubio's waffling answer I got the sense that the answer in his mind was no, but that he had to be non-committal to offending his base. Young earth creationists, as a group, tend not to be shy about their belief, and if he really thought the earth was less than 10,000 years old I imagine he probably would have said so. This isn't an issue like abortion or rape where saying your fundamentalist beliefs out loud can sink your candidacy. A substantial portion of Americans, and nearly all Republicans are still skeptical of evolution. I embrace evolution, but am skeptical that there is a real downside to him embracing biblical geology as a Republican candidate as long as he really believed it. The fact that he gave such a non-committal response to me suggests that he isn't a true fundamentalist at heart and/or he won't be making a genuine play in Iowa.