“So who was the first natural-born US President?”
I would guess the first president born after July 4, 1776!
That would be the first US president under the US Constitution that could be argued to have been retroactively declared “natural born” without dual citizenship to the King.
Martin Van Buren, #8, birthed in 1782 in Kinderhook NY, was the first American President not born a British subject nor of British ancestry (Netherlands).
But... the Revolutionary War ended in 1783 actually, so an argument could be made for John Tyler, #10... but he wasn't elected, he took over after William Henry Harrison (#9, born in 1773) died 1 month into office.
That gives us James K Polk, #11, born on November 2, 1795 in a log cabin in Pineville, North Carolina, as the first American President born on undisputed American soil and elected to the office.
Now for the first elected American President born to two American-born parents... :)
That is exactly why the Founders grandfathered themselves.
(“or a Citizen of the United States, at the time of the Adoption of this Constitution”)
This Grandfather Clause of Article II applied to the qualifications of 1) Washington, 2) Adams, 3) Jefferson, 4) Madison, 5) Monroe, 6) J.Q. Adams, 7) Jackson, and 9) Harrison, all of whom were born on British Soil, and to parents who were British subjects.
The first President not qualifying under the Grandfather Clause, but as a natural born citizen, is 8.) Martin Van Buren. Van Buren was born on Dec. 5, 1782, near Albany, NY, to two parents who were both U.S. Citizens.
Every predecessor to Van Buren was born British Subjects and with British allegiance through the British subjecthood of their parents.