I mean, take USA for example, is it even the same country after each amendment? After a Civil War? Civil Rights Movement? Landmark Court Rullings?
One could argue that Modern America began in 1965, with the passing of the Civil Rights Act. Or in 2001 when the Patriot Act was passed. Or in 2009 with Citizens United Court Ruling. Or in January 2017 when… you know. Or in January 2025.
When did the country really start?
Iran and Egypt
These are actually really interesting examples.
Take Iran:
On the one hand the area where it is located has had organized societies occupying it for an insanely long time.
This would make it one of the oldest countries by this definition.
On the other hand: when asked the question if the current Iran was the same country as back in the 1970’s, I would certainly say: “No.”
Occupied area and even some of the people might still be the same, but the cultural, societal and governmental characteristics have changed too radically to still be considered the same.
So, by this definition, Iran would be one of the youngest countries.
I think that uncertainty is what OP was getting at:
How much change do we allow to still consider a country the same as at a previous time in history?