Feel Old
Feel Old
[Megan and Cueball are walking.] Megan: Wanna feel old? Cueball: Does anyone? But OK, go ahead, I'm ready. Megan: This is the first presidential election in which there are voters too young to remember 9/11. Cueball: ...I wasn't ready.

This is yet another comic following xkcd 's recurring theme of using cultural or pop-cultural event dates to reference how much time has passed between two events, often with the stated intention of making someone realize how old they are. (See Comics to make one feel old as well as this xkcd blag post . However, this is the first comic where the title is actually directly related to feeling old!). In each case, the joke is derived from the shock that many adults feel upon realizing that events that feel relatively recent actually took place many years or even decades ago.
This becomes especially relevant when it is noted how old someone born during that time would be. Perhaps this is because for many people, there are fewer significant events and changes in their lives after they reach adulthood where there are no grade numbers and annual class changes to mark the continued passage of years. Hearing many years have passed framed in the context of the age of a child allows you to realize how long the period really is as you recall how much had occurred in your own life and how much you had grown by the time you were that age.
In this case, Megan makes Cueball feel old by noting to him that the 2016 U.S. presidential election will be the first U.S. presidential election in which there will be eligible voters who are too young to remember the September 11 terrorist attacks , in which hijacked airplanes crashed into the World Trade Center in New York City, the Pentagon  and a field in Pennsylvania in 2001. These attacks (commonly referred to as "9/11") were, in many ways, a defining event for an entire generation of Americans. This statement is made on the basis that the eligible voting age in the United States (the minimum age you must be to be eligible to vote in an election) is 18 years old, which is set by the Twenty-sixth Amendment to the United States Constitution .
Eligible voters in this election will have been born on or before November 8, 1998. The youngest voters will therefore have been nearly three years old (two years, ten months, and three days) on September 11, 2001. Megan is presuming that children under three years old (or at least some or many of them) were either too young to retain any memories from that age, or at least were too young to appreciate the significance of the attacks such that they would be retained memories 15 years later.
The previous U.S. presidential election took place November 6, 2012 and eligible voters would have been just shy of seven years old on September 11, 2001. Megan similarly presumes (or at least is generalizing) that voters who were seven on 9/11 were old enough to retain that memory through adulthood.
Particularly for those who were of voting age on September 11, 2001, it might seem startling that by election day, 15 years will have passed since 9/11. This might be particularly so given how significantly 9/11 shaped American society in the years following the attacks. 9/11 was a significant political point in the elections following the attacks as well as in non-electoral politics (such as discussions over homeland security, military actions, etc.) It is also unusual in that there was only one new Presidency (Obama replacing Bush in 2009) in a fifteen year period; historically, an average of nearly three new Presidencies begin in every fifteen years (the 44th Presidency is scheduled to end January 20, 2017, nearly 228 years after the first one began in 1789). To realize that there are people who have reached adulthood and weren't even old enough to be aware of 9/11 when it happened is a stark reminder of the passage of time since the event. The following election on November 3, 2020 , was the first in which there were eligible voters who were born after 9/11.
The 9/11 attack was already used in 647: Scary to make people feel old already back in 2009. At that time kids born after the event was old enough to discuss the event with adults which was what was scary for Rob in that comic.
When Megan asks Cueball if he wants to "feel old", he replies resignedly, suggesting that he recognizes (possibly based on the previous strips) that she's about to make him feel his age, but claims that he's ready. However, after she makes her statement, he admits that he wasn't ready. While he's technically aware of his age, that kind of perspective still catches him by surprise, and likely causes significant emotional discomfort. His additional discomfort may also be as a result of the serious and significant events of 9/11 Megan references in comparison to previous strips where lighter things like film release dates are cited (see the most relevant of those comics here: 891: Movie Ages ).
In the title text, Cueball asks Megan how long she can "keep this up" (i.e. how long she will continue to mention things to make him feel old). However, instead of addressing how long she can keep coming up with uncomfortable facts, she references a new fact that (intentionally or not) that likely has a similar effect of making Cueball feel old: That they're only likely to live another forty to fifty years (suggesting that they're both in their mid- to late- thirties). This may also be a reference to 493: Actuarial , where actuarial tables were used to estimate when (famous) people will die.
"How long are you going to keep this up?" Is also a question that is likely asked to Randall often. Perhaps, through this comment, he is confirming that he will continue making these comics until death or for as long as he possibly can.
[Megan and Cueball are walking.] Megan: Wanna feel old? Cueball: Does anyone? But OK, go ahead, I'm ready. Megan: This is the first presidential election in which there are voters too young to remember 9/11. Cueball: ...I wasn't ready.
