German history student here. I've talked to friends who study German about this and tested it myself. We can understand almost everything of Mittelhochdeutsch (~1050 - ~1350). The words look weird to us, but you can see where they're coming from and usually guess them right. Only archaic words are problematic. Now, the older state of the language is complicated. It's called Althochdeutsch (~500 - ~1050) and it's pretty abstract compared to modern German, but if you try really hard, you can get the gist of some sentences. Mostly because they sound almost similar to modern words, but not really. The syntax is different, too, which adds to the confusion. What's most important though is that you have to look at where the text is located. Since there were no rules of how to spell words, people wrote how they spoke, so their dialects made up a large part of what the text looked like. The same applies to Mittelhochdeutsch. So a German speaker with a strong southwestern dialect will find it easier to read a text in Althochdeutsch from the same area, but will probably have trouble understanding one from the northeastern part of what is now Germany. Generally, in a spontaneous conversation Mittelhochdeutsch would be hard to understand already, since you don't have the time to guess the words really. In written language, you will probably lose us at Althochdeutsch if we're not familiar with the respective dialect.