>> The average age of the world's greatest civilizations has been 200 years.

> Sounds like total bullshit.

You're right. It more like an avg of 118 years (median 73), not 200, depending on what you mean by "greatest" ofc. The Ancient Romans, US, and Dutch pulling that average up and 1/3 of historically modern continuing democracies having a short lifespan being founded around WW2, pulling it down.

The Roman Empire was not stable for an uninterrupted period of time. It's conceptually called a single state for simplicity, because such a large period of time and area of land naturally had instability (less than current Europe, granted). The Roman Empire democratic government lasted about 487 years before falling into autocratic rule.

republican government. I wouldn't call that a democracy, not in the modern sense of the term.