The term "United States" existed long before the Constitution was ratified. Article II Section I Clause V states:
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.
All who were in the United States at the time of the adoption of the Constitution were therefore, eligible.
You are correct. However, I think the author meant that since they were not ‘natural born citizens’ they would not be eligible ...
thus the second clause in the Constitution ... “... or a Citizen of the United States, at the time of the Adoption of this Constitution ...”
It is good that the second clause was added by the Founding Fathers. Back in April 1789.
Maybe Obama is really old? : )
And indeed, there were two Presidents who were born after the United States came into existance, but before the Consittution was ratified. Presidents Martin Van Buren, born in December of 1782 and Zachary Taylor, born in November 1784.
But the period was not really all that long. July of 1776 (at the earliest, some might date the existance from ratification of the Ariticles of Confederation (March 1781), or the Treaty of Paris(signed September 1783, ratified in January 1784) to June 21, 1788 when the Constitution was ratified. I used the 1776 date, but you'd get the same result if you used the 1781 date of the Articles. I would not use the Treaty of Paris, since that was only getting the Brits to aknowledge what had already been created.
But it really doesn't matter because both were Natural Born Citizens of the United States and were also citizens at the time of the Adoption of the Constitution, as was everyone then living in the United States that did not choose to evacuate to Canda or England.
But other than those two, who could hardly be considered founders, the rest of the first 12 Presidents, where not natural born citizens of The United States. They were natural born subjects of King George of England, Although all were born in the King's American colonies, it would have been possible for an immigrant from either another British colony, like Alexander Hamilton who was indeed a founder, to have become President during that period, as long as he was living in the US at the time of the adoption of the Constitution. So could Friedrich von Stueban,a Prussian by birth, but who was the first Drillmaster of what was then the Continental Army. He became a citizen in 1783, before adoption of the Constitutioin. (But virtually no one wanted him to be President, probably not even himself)