The United States as a nation-state is older than almost all of European nation-states, barring a few. It is one of the few nation-states that has continually existed since its foundation, without interruption.
Germany for example was unified in 1871. Were there 'Germans' (who spoke the language) before then? Sure. Germany? No. Neither Prussia nor Austria is the German nation-state. The modern German state was recreated after WW2 in 1940s. France is on its god knows what number of Republic now after Kingdom-Republic-Empire-Republic revolutions etc (although I'll grant that 'France' as a singular 'country' is older than the US). Italy was unified well later. The dutchy of Poland is not the modern Poland, etc.
It is one of the oldest(among the other very few) continually existing nation-states still alive today.