On which page in the US Constitution it says “a natural born citizen is one whose parents are US citizens”?
I had to memorize the entire US constitution and all amendments for my citizenship test back in 1970.
It doesn't say that a "Natural Born Citizen" is one whose parents are U.S. Citizens at the time of his birth. But...it says this....peculiar only to the presidency:
U.S. Constitution [Article II, Section 1, Paragraph 4]
No person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty-five Years, and been fourteen Years a Resident within the United States.
When the framers wrote the Constitution this "Natural Born" provision was commonly known the same way we, today.....know that the "World Series" pertains to baseball....and the "Super Bowl" pertains to football. One does not have to say.....the "Super Bowl Football Game" because everyone knows that the "Super Bowl"......IS Football! By including "Football Game"....you're just being redundant!
"The Law of Nations", written mid 18th century (1758) was the initial description of the term, "Natural Born". It was penned by "Emmerich de Vattel" and read by statesmen throughout Europe and the Colonies and quoted frequently by the framers. Here is the pertinent section, #212:
“The citizens are the members of the civil society; bound to this society by certain duties, and subject to its authority, they equally participate in its advantages. The natives, or natural-born citizens, are those born in the country, of parents who are citizens. As the society cannot exist and perpetuate itself otherwise than by the children of the citizens, those children naturally follow the condition of their fathers, and succeed to all their rights. The society is supposed to desire this, in consequence of what it owes to its own preservation; and it is presumed, as matter of course, that each citizen, on entering into society, reserves to his children the right of becoming members of it. The country of the fathers is therefore that of the children; and these become true citizens merely by their tacit consent. We shall soon see whether, on their coming to the years of discretion, they may renounce their right, and what they owe to the society in which they were born. I say, that, in order to be of the country, it is necessary that a person be born of a father who is a citizen; for, if he is born there of a foreigner, it will be only the place of his birth, and not his country.”
This term, "Natural Born Citizen" was commonly accepted throughout the world as what the above paragraph indicates the definition to be. It has only been recently (last 50 years or so) that this definition has come into question. This is due to the fact that the U.S. Constitution is no longer taught in our public schools.