Your reading comprehension is lacking.

I was referring to current governments, and even noted that revolutions change that.

"Aquisition of Sovereignty" doesn't fit that definition.

"Date of last Subordination" is closer, but still not correct (US is #3 oldest in this category btw)

Check the 2nd section titled "List of countries by current form of government". Aka, exactly what I was referring to. And it turns out that the only country the US government is younger than is San Marino.

Turns out that colonialism, 2 World Wars, and the Cold War weren't particularly conducive to longevity of governments