Jackson was 22 when the Constitution was ratified. He was grandfathered in under the Article II Section 2 exception "'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..."
His parents were original citizens of the United States at the founding, since they were already here as colonists (in 1765) when the Declaration of Independence was signed.
-PJ