According to the Constitution, the President must be at least 35 years old. Theodore Roosevelt is the youngest person to have become President. He succeeded William McKinley, who was assassinated. Here are the youngest Presidents and their ages when they took office.
1. Theodore Roosevelt 42 years, 322 days
2. John F. Kennedy 43 years, 236 days
3. Bill Clinton 46 years, 149 days
4. Ulysses S. Grant 46 years, 311 days
5. Grover Cleveland 47 years, 351 days
SOURCE: WHITEHOUSE.GOV