Other nations have had several advantages. Smaller populations, making experimenting and changes easier. Ability to see what the U.S. did wrong and learn from its mistakes. And so on. Democracy is at at least version 3 today, but the U.S. is stuck at version 1.0 and will be for the foreseeable future.
To put it another way, the U.S. Constitution is mostly from 1789, while the Swiss Constitution is from 1999. Does anyone think there might have been some advances in the science of democracy over those 210 years?
The basis for the USA (ie the Constitution) has changed little in 200 years but the government has radically changed.
Look at the Supreme Court decision in the 20s that decided that constitutional limitations on the federal government also applied to the states and local government. Around that time an idea took hold among many that the constitution was a "living document", meaning that it's meaning changed with the times and it was OK to decide what the founders intended but never said.
Basically the constitution means whatever you want it to mean.
So the issue isn't that the US was an early iteration of democracy, it's that the judicial branch is out of control and the legislature has bought into the idea that's its ideas extend to things the constitution says nothing about (eg the federal Department of Education).
The only issue is that modern politics in America is a lot more like trench warfare, with lines drawn and and fixed. Opinions on every issue fall along party lines, with a few exceptions.
Given how dysfunctional our political system is, I'll take an outdated and flawed constitution before letting the clowns in power now try and write a better one. Sure, maybe it's not vitally important that the Constitution outlaw the government quartering troops in our houses, but I would not like to see the people behind the Patriot Act, Guantanamo Bay, "extraordinary rendition", and internet wiretapping get a shot at "revising" the fourth and fifth amendments.
As far as what century it applies to, the problems of government are pretty timeless. When one class of people is free to exploit another, the results are the same no matter what century is it.
California does not have referendums required to approve legislative action.
California doesn't allow you to leave a municipality to join or create another.
California does not have proportional representation.
The key is not letting people decide everything for everybody, but preserving the individual's ability to decide their own fate.
The developing "democratic" disaster in the US is not what the founders wanted. They thought they had put things in place to prevent this, but they failed. And its looking like only a major crisis that our leaders cannot cover over will force us to reconsider how we operate.