I realize there are numerous denominations of the Amish, but I'm wondering whether any or all of them believe in the literal, six-day creation and a six-thousand-year-old earth. I've looked in many places but haven't found anything definitive.
I'm Amish -- sort of.* You are correct that there is a lot of variation within the Amish tradition (and even more if you include Mennonites, another branch of the Anabaptist heritage), so I can't speak for all. However, I think I'd be fairly safe to say that many, if not most, of us believe generally in a young earth and a literal six-day creation.
Historically, the issue was not a major focus, especially since Amish and conservative Mennonite involvement in higher education was (and still is) fairly limited. However, even the Amish and Mennonites have been influenced by fundamentalism, with its insistence on Biblical literalism and rejection of "modernism." Organizations like Answers in Genesis have had a large influence on many conservative Anabaptists.
* I'm a member of the Beachy Amish-Mennonites, an offshoot of the Amish. There is quite a bit of variation within the "Beachys" as well, but we all have cars and electricity and phones. Many of us utilize the internet regularly and live fairly normal American lives, while still maintaining some degree of distinctive dress and lifestyle. (The Mennonite Encyclopedia Online has more about our history and name, while this very unofficial but fairly informative site has some pictures, FAQs, and more.