The first Natural born citizen President was Martin Van Buren 8th President of the U.S. Both of his parents became citizens when we became a nation. Hence he was a natural born citizen per Article 2. The first President to be born to Americans citizens “ born after” our nation was formed ( and Natural born citizens themselves) would be Polk our 11th President. hope this clears up your question?
The author stated Polk is the first Natural Born Citizen elected President.
His parents were born in North Carolina.
Natural Born Citizen: Born to parents who were born in the US.
Van Buren was born in 1782 so he was qualified to be president for the same reason Washington and Jefferson and Jackson were - citizen of the U.S. at the time the Constitution was ratified.