You could argue that modern Japan only exists since WWII. The major changes required after losing in WWII majorly changed the country.
You could also argue that the US is a new nation since the Civil War, so it's 160ish years old. If you ignore the civil war, what about when various states were added? Does the fact they were added gradually rather than all at once mean it's the same country? It's hard to argue that a country that was founded on the idea that all land-owning white males should get to vote is really the same as one that in 2022 believed that any citizen of any race or sex over the age of 18 should get a vote. Though, I suppose in some ways 2025 USA is showing it's still the same country as 1776 USA.
It's all pretty arbitrary though. What defines the start and end of a country? Does changing names count? Does changing borders? How radically does a government have to change to mean it's a new country? How radically do founding documents need to be changed? I guess it's the Country of Theseus. When is it no longer the same one?