They don't have any evidence, they just have un-tested hypothesis from what I've seen. Also, there is no scientific evidence supporting a young earth, there is science that still holds up with a young earth but none that supports it. The only plausible explanation for a young earth is that it was created with the appearance of age, so God basically screwed with us. Just research a few geologic topics and you'll find plenty of evidence for an old earth. Even the 6 day creation described in the Bible does not add up, insects came before humans. Days could easily mean periods, or eras, or maybe eons? No where in the bible does it say the earth is 6000 years old, humans decided on that later by making a chronologic time scale of the writers. Regardless, the most important book in the world does not include metaphors, you don't have to put any actual thought into what you're reading (oh wait maybe when it says day that may mean a much longer time than what we now call a day)? A day length is not even universal, every planet in our solar system has a different length of a day, one has a longer day than year.