The American fascination with the British royal family...

[Taunton Blue Genie]

I know this isn't the point you are making but actually the nation state that is the United States is one of the oldest on earth now.

It was just a whimsical comment, as you guessed. Like many developed countries, the population there is ageing, of course.
Japan is a particularly interesting example as immigrants are not really fully integrated to there.