Stating the obvious but...
Young Earth Creationism is not scientific. Not just because it's wrong but also because it's not primarily trying to appeal to science when it claims that the Earth and the universe are not as old as they seem. Rabbi Slifkin in his recent "disagreements" with Dr. Betech has taken a rather reasonable approach as far as refusing to engage in a scientific debate since the problem is not really about science.
Young Earth Creationists make pseudo-scientific arguments. But this is just a red herring. Ultimately it all boils down to the fact that they respect the authority of scripture and in Judaism the authority of "Daat Torah" and Chazal more than the results of scientific inquiry. In an earlier post about the Documentary Hypothesis I noted a similar phenomenon i.e. that opponents of the Documentary Hypothesis have no problem dipping their feet into both camps - criticizing the literary methods of the DH'ers on one hand (with Cassuto's help) and appealing to the special ineffable, Godly nature of the Pentateuch on the other hand. YEC's often (but not always) engage in similar behavior asserting the authority of scripture and revelation over science but not shying away from "beating the scientists at their own game" by engaging in pseudo-scientific arguments. Ultimately, though, I think we can assert with some confidence that YEC's are not basing their views on "science" as much as they are being motivated by the infallibility of Chazal/Torah.
At the end of the day it's pretty hard for an Orthodox Jew to argue with YEC's since one has to tie one's hand behind one's back and admit that the Torah is the word of God. The best you can do is show HOW there are methods of interpreting the Torah less literally or appeal to the authority of Rishonim and Achronim who held similar views. But as I've said before either way you look at it, God was a little misleading somewhere - either when he wrote the Torah so obscurely or when he created the world to look like it was older than it was. And how exactly can you or me or anyone determine which of the two sources of truth should be the one to be "taken non-literally"? At best all one can show is that there is a long tradition of people who didn't take the Torah literally or that believing in science over a literal reading of the Torah is not
kefira. But I don't think there really is any good way of saying to the YEC's "you are wrong and I am right and this is why!"
Ela Mai? The only real way to be
machria the problem once and for all is to demonstrate, by appealing to the simple lack of evidence, that the Torah is not the explicit word of God and is not a source of truth comparable to science. The Torah is, as far as we know, just a book like all other books written by humans - trying to figure out the answers to life, the universe, and everything.