I'm studying Chinese (Taiwanese style, so traditional characters), and my understanding is anything back to about the Han Dynasty (~200 BCE) is intelligible to an educated Chinese speaker.

Resiliency is one of the weird beneficial side-effects of having a writing system based on ideas instead of sounds. Today, you've got a variety of Chinese dialects that, when spoken, are completely unintelligible to one another. But people who speak different dialects can read the same book just fine. Very odd, from a native English speaking perspective.