"I am not going to exploit for political purposes my opponent's youth and in experience."
So went Ronald Reagan's famous line in one of his 1984 debates against Democratic challenger Walter Mondale.
Will Biden have a similar quip ready on Thursday?
OK, Biden is not nearly as old as Reagan, who was 73 at the time of his re-election. But at 69, he has almost three decades on Ryan, his 42-year-old debate opponent.
Biden is also known for his exuberance and devilish charm, so perhaps the age gap won't be as apparent as it was in 2008, when 47-year-old Barack Obama shared a stage with 72-year-old John McCain. Or as it was in 1988, when 67-year-old Lloyd Bentsen debated his boyish vice presidential foe Dan Quayle.
Stylistically, their birthdays might not matter much. But the age difference neatly crystallizes the arguments being presented by both campaigns.
Will voters choose experience and decide to move forward on the path we're on? Or will they look to a pair of fresh-faced outsiders who offer a new direction?
5. Can Ryan be seen as commander in chief?
Just as Mitt Romney needed to prove last week that he's acceptable to Americans to serve as president, Ryan needs to convey that he's ready to fill in as president in case something happens to Romney. It's one of the top tasks for the challenger's running mate in a vice presidential debate.
We'll be keeping a close watch on Ryan to see if he touts his ability to step in, if needed, to the leading role.
The Romney campaign is confident that the congressman from Wisconsin will pass the test, thanks to Ryan's position as House budget chairman and his mastery of complex fiscal issues.
"It will come down to his competency on the issues, his command of the issues," a Romney campaign official told CNN.
But Republican strategist and CNN contributor Alex Castellanos thinks that this time around, passing the competency test is not as important.
"There is very little pressure for either of these vice presidential candidates to prove they are ready to step into the office of president. Both their bosses are relatively young and in great health," Castellanos said.