All countries are similarly young - nationalism itself is a fairly recent invention.
I say, young man, wot?
Germany, France, Spain, Italy, England, Sweden, Austria, Russia . . . all countries with several times the years of the US . . . those are "similarly" young?
In geologic time, maybe.
In terms of modern civilization, not so much.
US is less than 300 years old.
All the ones I mentioned above have easily a thousand years of modern and pre-modern history behind them. I don't get where this "similarly" comes from.
It may be true that nationalism is historically "recent" but European countries have been having nation-on-nation wars since around 1,000 AD, give or take a couple of hundred years.
They refer to themselves -- often smugly -- as the "old world."
In a hundred thousand years of human history, fraught with dynasties of this and that kind, a mere thousand years isn't a lot.
Unless, of course, your country didn't officially get off the ground until the mid-late 1700s.
Yet in spite of this, we're that "young upstart nation" with a crusty and crumbling Constitution document that's too old to be of any real value.