[SapporoChris]

https://en.wikipedia.org/wiki/List_of_sovereign_states_by_da...

Highly debatable. However, I think most citizens would consider their nations age date of acquisition of sovereignty, not from the date of the current form of government or any other standard.

Anyway, calling United States one of the oldest countries in the world is about as defensible as calling it one of the newest countries in the world. I just don't see the data for it.

[peteradio]

There is no data for it, its definitional. The two definitions skew the perception of the answer. Perhaps you care about stability and somehow you've got a correlation between length of government standing and your perception of stability. What defines a country?