None of the Founding Fathers were natural born citizens. They were all born as British subjects, hence the grandfather clause in Article II, Section 1.
Martin Van Buren in 1837 was the first president who qualified as a natural born citizen.
"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."
'at the time of the Adoption of this Constitution' refers to 'or a Citizen of the United States' and covers the founders at the time of the adoption of the Constitution.
as all such persons are no longer alive, A2S1 could be rewritten without changing effect as:
"No Person except a natural born Citizen 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."