Old Mandarin vs. Mandarin Chinese? How intelligible is Old Mandarin to the speakers of Mandarin Chinese?
If an Old Mandarin Chinese speaker heard a Mandarin Chinese speaker or vice versa, how fluent would the communication be?
I believe the situation is more or less the same in all languages, that the language drifts across generations. Whereas you could easily communicate with Isaac Newton, it would be a little more difficult to do with Shakespeare, and more so with Chaucer. When you get to Richard Lionheart and earlier (say, the times of Beowulf), you can just forget any meaningful exchange of ideas.

As for Mandarin, you could speak perfectly well with late Qing people, although they would still retain tones that are now forgotten. Going back to Cao Xueqin's time (1700s), you would still be able to discern most of the vernacular used, just like you can read 红楼梦 without too much trouble. But you would get progressively lost talking to Su Dongpo and Princess Wencheng of Tang, they would speak a completely different language.

The thing with Chinese, though, is that you could still exchange ideas in writing with these people.