Its nonsense. China as a national and cultural entity is not 76 years old. Changing constitutions does not make it a different country,
Yes, it does. Country and Nation are not synonyms. Country refers to Government while Nation is about culture.
China as a Nation, that is a people with a shared culture and languages, is thousands of years old. China as a Country is less than 100 years old.
...it is only americans who adhere to that belief system because their country didnt exist prior to their constitution.
Since you are presumably not American how would you answer this question: How old is Panama?