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There really is a grandfather clause in Section 1, Article 2:

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.

I seem to recall that John Tyler was the first President who wasn't grandfathered in, but I may be wrong. It's been a couple decades since my Constitutional Studies classes in college.

Cheers!

You were close. Martin Van Buren, the 8th President, was the first President born a US citizen at birth:

From Wiki: Martin Van Buren (Dutch: Maarten van Buren pronunciation (help·info) ; December 5, 1782 – July 24, 1862) was the eighth President of the United States (1837–1841). Before his presidency, he was the eighth Vice President (1833–1837) and the tenth Secretary of State (1829-1831), both under Andrew Jackson. Van Buren was a key organizer of the Democratic Party, a dominant figure in the Second Party System, and the first president not of British or Irish descent—his family was Dutch. He was the first president to have been born a United States citizen,[2] his predecessors having been born British subjects before the American Revolution.[3] He is the only president not to have spoken English as his first language, having grown up speaking Dutch,[4] and the first president from New York.

I was born in America to an American father and an Italian mother. My mother naturalized when she was 50 years old, and I was 13.

I have never considered myself eligible to be President.

I can be a Senator, like Cruz, or a Congressman, but not President.

I don't have a problem with that.

John Tyler was the first President born after the adoption of the Constitution. Ironically, his father voted against adoption of the Constitution at the Virginia convention which narrowly voted in favor of accepting the Constitution. By the wording of the Constitution, it was not clear if he became President on Harrison's death or merely "Acting President." (That was finally addressed by the 25th amendment in 1967.)

Martin Van Buren was born in 1782, after the Declaration of Independence but before the Constitution was adopted.