Who was the youngest president ever to have served in office?

Answer 1

Theodore Roosevelt was the youngest president when he took office after McKinley died. He was 42 years and 322 days.

John Kennedy was the youngest person elected President.
1. Theodore Roosevelt 42 322 2. John F. Kennedy 43 236 3. Bill Clinton 46 154 4. Ulysses S. Grant 46 236 5. Barack Obama 47 169 6. Grover Cleveland 47 351 7. Franklin Pierce 48 101 8. James Garfield 49 105 9. James K. Polk 49 122 10. Millard Fillmore 50 184
The youngest ever elected was John F. Kennedy, who was 43. The youngest man to actually become President, however, was Theodore Roosevelt who, at age 42, became President after William McKinley was assassinated.
"At 43, (43 years, 7 months, 22 days) John F. Kennedy was the second-youngest president-elect in U.S. history and the first Catholic to become elected President."

(See: http://abcnews.go.com/Politics/President44/popup?id=6640072&contentIndex=1&page=8&start=false)

Theodore Roosevelt (42 years, 10 months, 18 days) is the youngest President Elect.

See: http://en.wikipedia.org/wiki/List_of_United_States_Presidents_by_age
John F. Kennedy was the youngest person ever to be President-elect (43 years 236 days old)

Theodore Roosevelt was younger than Kennedy when he first took office, but

he first took office due to the death of President McKinley. When he was elected to another term he was older than Kennedy was.
John F. Kennedy, 35th US President, was 43 years of age when elected. He was the youngest ELECTED US President.
John F. Kennedy at 42 was the youngest president to be elected. Theodore Roosevelt was younger when he first took office, but he assumed the office of William McKinley who was assassinated. When he was elected to a term of his own in 1904, he was older than Kennedy.
John F. Kennedy was the youngest president ever elected (43 years 236 days old).

However, Theodore Roosevelt became vice president at a younger age (42 years 322 days old) and became president after McKinley's assassination and was technically a younger president than JFK.
sadam hussain and he was negative 865,634,384,653,826,486,482,864,264,623,864,868,685,683,264,749,764,036,765,999 years old
John F. Kennedy at age 43 was the youngest person to be elected U. S. President, but when William McKinley was assassinated, Theodore Roosevelt, age 42, became the youngest person ever to assume the office of the U. S. President.
Theodore Roosevelt was the youngest president, but JFK was the youngest elected.
John F. Kennedy was the youngest president to serve. He became the United States president at the young age of 43.
The youngest U.S. President was Theodore Roosevelt, who was 42 when he assumed the Presidency on September 14, 1901.

Here is a top ten list of the youngest U.S. Presidents to date:

1. Theodore Roosevelt (42)
2. John F. Kennedy (43)
3. Bill Clinton (46)
4. Ulysses S. Grant (46)
5. Barack Obama (47)
6. Grover Cleveland (47)
7. Franklin Pierce (48)
8. James A. Garfield (49)
9. James K. Polk (49)
10. Millard Fillmore (50)

Answer 2

It depends on your definition of "leave office." If you mean just stop being president, then that would be John F. Kennedy. He was 46 when he was assassinated. James Garfield was 50 when he was assassinated. But the youngest person to get elected, serve his full term, and exit office alive was Theodore Roosevelt. He was 51 years old when he left.

Answer 3

The youngest president to die in office was John Fitzgerald Kennedy, I believe. :)