The phrase "depending on how you want to measure it" is doing a lot of work in this post, the rest is American exceptionalism. San Marino has a constitution that edates back to 1600. Iceland has had a parliament for 1,000 years. Even leaving aside the fact that many of the world's democracies were deposed by the US government in the 20th century, the US has neither a notably long-lived nor notably democratic system of government. What it has is a huge military budget, and that enables it to seed governments in the way that it chooses.