This is one of the things I studied heavily in graduate school, and I wrote my thesis over a text from the Anglo Saxon (Old English) period. I feel qualified to answer this question.

The first thing you have to understand is that language changes over time - there is not an event that changes language drastically from one moment to the next. In reality, the language underwent centuries of mingling and gradual change. Many influences culminated to make English what it was in and around this time period. Germanic, Celtic, and Roman influences mingled in the land that we call England before 500-800 AD, after which Scandinavian influences entered into the language from viking sources. We still consider it all "Old English" through this time period. The definitive event (because we have to draw the line somewhere) that dictates the line between Old English and Middle English is in 1066, when William the Conqueror invaded "England" and made French the official language of state. This brought in substantial changes to the language and made it sound far more similar to the language we have today.

Of course, it still wasn't quite there. This only takes us up through Middle English, and Shakespeare's English was Early Modern English. Someone else will need to come in and talk about that time period, though. I'm tired, and that's not the time period I have the most experience studying.

In other words, language change is based off of the specific events and gradual prestige and usage of a language over time, and no two languages have the same "experiences."