First take a good look at this picture:

This picture was taken in late 1991. It shows the then President George Bush (Sr) along with all four of his living predecessors: Reagan, Carter, Ford and Nixon. All five of them have autographed the original photograph. This original, along with a certificate of authenticity, can be purchased for USD 7,500 (about INR 470,000). See below:

If the incumbent President Barack Obama were to have a similar opportunity, he could also be accompanied by all four of his living predecessors. Only two of the five in the above picture survive, having crossed 90.
As of January 2015, there are four living former presidents:
President |
Term of office |
Date of birth |
Jimmy Carter |
1977–1981 |
October 1, 1924 (age 90) |
George H. W. Bush |
1989–1993 |
June 12, 1924 (age 90) |
Bill Clinton |
1993–2001 |
August 19, 1946 (age 68) |
George W. Bush |
2001–2009 |
July 6, 1946 (age 68) |
Although George Bush (Sr) also had a W in his name, no one thought of calling him Dubya.
An interesting coincidence is that Carter and Bush (Sr) were both born in 1924, while Clinton and Bush (Jr) were both born in 1946. Prior to Carter, both Nixon and Ford were born in 1913.
As in the case of the British royal family, every possible bit of trivia has been teased out of Presidential history. For instance, there have been times when 5 ex-Presidents were living. The last such instance was between January 2001 (George Bush (Jr)’s inauguration and June 2004 (Reagan’s death), with Ford, Carter, Reagan, Bush (Sr) and Clinton. And there have been some occasions when there were no living ex-Presidents. Obviously this was true of George Washington’s presidency in 1789-1797 but it occurred a few more times, the last occasion being between February 1973 (Johnson’s death) and August 1974 (Nixon’s resignation). A listing of the number of living Presidents and ex-Presidents at any point of time since 1789 can be seen here:
http://en.wikipedia.org/wiki/Living_Presidents_of_the_United_States
Throughout the 8 years of Obama’s two terms, the 4 ex-Presidents were living. When Trump succeeded him in January 2017, Obama become the 5th ex-President co-existing with him. This is a record, though it has happened before a few times. Details can be seen from the link above.
In the US, any aspiring school quizzer worth his or her salt would have to know the names of all Presidents and their terms. By college level, they would have to know the Vice-Presidents as well. This comes in useful because there are enough obscure Presidents and Vice-Presidents to generate many questions. In connection with Vice-Presidents, Woodrow Wilson is believed to have said “There were two brothers. One went to sea and the other became Vice-President. Neither was heard of again.”
Indeed, relatively few Vice-Presidents are elected to the Presidency immediately after their Vice-Presidency. Bush (Sr) was the first to do so in over a century. Richard Nixon had a gap of 8 years before he was elected President.
Now, if you are from India, you only have a data set starting from 1947 (and 1950 for Presidents and 1952 for Vice-Presidents). See if you can recite from memory all the Presidents, Vice-Presidents and Prime Ministers (Yes, Gulzarilal Nanda also counts). Toss in the few who have been designated Deputy Prime Minister. (Not in UPA-1 or 2 or at present).