That's not a joke answer. Keep reading.

So why are we able to say that year is the end of it? Because in reality the distinction between Old English and Middle English isn't an objective reality. The period of Old English ends with the Norman Conquest because at that time we start to talk about Early Middle English, being the language spoken in the area under influence from the Normans.

All languages are always changing. Your English is different from your mother's English and hers is different from her mother's (assuming they were English speakers). That's the reason you have places like Ontario and Michigan which speak almost identically (except around the border where they're actually more different from each other, not less) but not actually identically. Someone knowing what to listen for can pick out the differences.

When we say "This is Early Middle English, but this here is Late Middle English", those are based on features of the language, but they're not concrete realities. They're just labels we've given based on ultimately arbitrary lines in the sand.

Of course in some cases the lines are much less arbitrary because there was some major historical even which caused the language to start changing. In 1127 the Song moved their capital to Lin'an, thus causing a major change in the way the local Wu language was spoken thanks to their Mandarin influence. That's a major language-changing event. For Albanian, Enver Hoxha changed the official dialect from Gheg to Tosk in 1944 when he came into power.

In the case of English, it was 1066.

Did they suddenly start speaking completely differently than they did in 1065? Not at all. It takes time.

But "When did the last speaker die" isn't the right question.

Old English didn't die. Latin didn't die either. They just became something else. Your great great grandfathers English (again assuming he spoke English) didn't die. It just became the next generation's English, continuing on town to you.

So we aren't really saying that me and my grandpa spoke different languages (we call them both English) even though we did (they were very different Englishes), but we have to draw the line somewhere if we want to be able to abstract things to be able to talk about them in meaningful groups, so,

tl;dr: 1066.