I think this is a very good question, and one that - in slightly different form - I had thought about recently. I had come across a couple of events that I referred on for moderation, while thinking that I would not like to be the moderator having to make a final decision; so I completely understand WW. Here are some thoughts.
(1) The event in question: In my opinion, complaining about the presence of a common swear word in a comic illustrating a point is unnecessarily prudish, in particular considering that this isn't only a site for US participants living in a ludicrously PC world (I remember an excellent onion article on "NippleGate a year later"). Of course it depends on the comic, which I have not seen (I hope it wasn't xkcd which certainly is always appropriate by any reasonable standard). This is very different from such language chosen by a user submitting a post, which is never necessary, and probably almost always a reason for me to consider flagging a post for further review. I live in NYC, and drop the F-bomb once every other sentence. But a user wanting to participate in a public discussion on mathematics should only do so if (s)he is mature enough to realize there is a difference between oral conversation, and talking to others online: without the context of gestures, mimics, and such, words on digital paper take on a different dimension. This isn't YouTube, and it shouldn't be.
(2) Age of user: This is a public forum with fairly professional standards. Age shouldn't prevent anyone from posting here, but if you post here, live up to some common standards reasonably expected by a community. At 13, I was wildly immature, but wouldn't have posted in a public discussion my argument peppered with swear words. I read the 13 cut-off as meaning "being old enough to have a mathematically interesting question, and mature enough to ask this question as you would in class to your teacher." In fact, why should a younger math prodigy able to do both not participate - rather than a college kid who vomits a question out in an aggressive, demanding fashion? I assume this has legal reasons to allow for certain standards here, but even PG-13 is a US concept of fairly little meaning in Europe or Asia.
(3) What does civility mean: For me cusswords are much less of a problem than more difficult to pinpoint lack of civility. They are easy to pinpoint and remedy as needed. I have noticed a fair amount of lack of civility in more subtle ways. By way of summarized examples: (a) a user posting an answer that was sarcastically mocking another user who had posted an earnest, civil question (how do you prove it was sarcastic?); (b) users posting with zero punctuation, all lower case, skipping vowels (as, for some, this is a cool way of online communication - I confirmed that with those much younger than me if it isn't obvious enough) writing in incomplete sentences, or those that don't even match their point (this is a big pet peeve of mine: English is my 3rd language and I was terrible at it in middle and high school, and these posts are typically by native speakers (I would never complain about an obviously non-native poster who tries) showing what feels to me as an extreme lack of courtesy and a pitiful sense of having to project a 'cool' online image which strikes me as dismissive. You can take the position that these posters can't do any better, are all non-native speakers, and where is my evidence; but, please: it is obvious); (c) questionable user names - I remember one describing sexual excitement, and one used, per urban dictionary, as a derogative word for "hairy foreigners resembling simians" (but where do you draw the line? MSE shouldn't be a police state either and allow for creative expression). It may be just me, but what I summarize in (3) rubs me the (very) wrong way, because it is more subtle but just as annoying, and completely unnecessary. On the other hand, except for extreme cases, I would not want to be the moderator having to make a call.