"We’re a young nation, and like any adolescent, nothing rouses us to fits of bitter delight more than detecting hypocrisy in others [i.e., Jefferson]."I disagree. You can define "nation" different ways. One looks at the government and the land included in the nation, another at the "people", an approach which has lost favor. (Hitler conflated the Reich and the German people.)
Nations like France, Italy, and Germany really were formed in the 19th century, as a government consolidated control over its territory. "England" and "Britain" form something of an exception--England has been consolidated under the crown since the Norman Conquest, give or take the odd civil war. Britain still isn't sure whether it includes the Scots or not.
There are nations which have been around in some form or other for centuries: China, Japan, Ethiopia. And if you allow for boundaries to change, Russia, Persia/Iran, Egypt, and others can qualify. But when you look Latin America, Africa, Australia, and South and Southwest Asia you see the impact of colonialism and nationalism, recent phenomena.
So I repeat: we aren't a young nation, considering the histories of all the nations in the world. The young nation meme is just a way to claim American exceptionalism.