Not all Countryhumans make it to adulthood.
Countryhumans are born when people theorize about their existence. This means that they can poof in and out of existence.
If there are thousands of people who think Australia-Hungary is real or could become a real country, then a Countryhuman child is born to Australia and Hungary.
Since thousands of people think Scotland should be a country, there is a child Countryhuman Scotland running around somewhere.
But this also means that child Countryhumans are not immortal. They can die and not go to Pangaea. If the process to create their country is stopped and ended somewhere along the line, they die. They can be reborn the same or slightly different anywhere from days to years later, but they still died.
