How many presidents took office at younger ages than Barack Obama would?

a) 2

b) 4

c) 7

d) 10

Answer: b) Obama — the first-term Illinois senator who is the Democrats’ presumed nominee for this year’s presidential election — would become the fifth youngest president at the time of inauguration if he is elected.

Obama, who celebrated a birthday on Monday, would be 47 years and five months old on the inauguration day of Jan. 20, 2009. The oldest presidents younger at their swearing-in were:

• Theodore Roosevelt, who was a month and a half shy of his 43rd birthday when he succeeded assassinated President William McKinley in September 1901.

• John F. Kennedy, whose November 1960 victory made him the youngest elected president when he was sworn in the following January at 43 years and about eight months.

• Bill Clinton, who was elected in November 1992 and sworn in the following January at age 46 years and six months.

• Ulysses S. Grant, who was elected in November 1868 and sworn in the following March at age 46 years and 10 months.

George W. Bush, the current incumbent, was 54 years and six and one-half months old when he took office in January 2001.