An old One Big Happy strip that’s been hanging around on my desktop for a couple of years. When you go to explain why it’s so weirdly funny, it turns out to be a complex exercise in what’s known in the linguistics trade as quantity implicature: someone uses a quantity expression, like 6 people or 18 years old, and we understand the speaker’s intentions to be to suggest exactly that quantity, or at least that quantity, or no more than that quantity — in technicalese, we take the speaker’s words to implicate one of these things — depending on the context and our assessments of the speaker’s reasons for mentioning that quantity in the context.
The standard discussions of quantity implicature are about reports of states of affairs. If, for example, a well-intentioned speaker tells you that there were 6 people at their birthday party, you take them to be conveying that there were exactly 6 people. I mean, if there were 8 people at the party, it would be true that there were 6 people; but then it would be uncooperative to say that there were 6 people, because if you knew there were 8 you should and would have said so, therefore saying there were 6 implicates that there were exactly 6. (This would be a good time to take a deep breath and rest for a moment.)
Now to the OBH strip. To start with, it’s not about reports, but about requirements, about stipulated criteria — things like
You must/should be this tall [pointing to a measure stick or mark] to get on the ride.
Which absolutely does not require that you be exactly that tall, instead that you be at least that tall: that tall or more (it sets a lower bound), Similarly,
They have to be 18, or it’s statutory rape. [18 or older]
You need to have four pieces of identification. [at least 4]
There are stipulated criteria that we understand to be exact, but they need very special contexts. As in a likely understanding of:
You must be born on September 6th to collect the birthday jackpot.
And finally, there are stipulated criteria we take to be requiring that something be at most some quantity: no more than that quantity (they set an upper bound):
You may/can (only) take 4 pieces of baggage with you.
(which permits you to take, say, 2 pieces of baggage, but not 5). Or in a negative formulation:
You can’t take 5 pieces of baggage with you. [but 4 or less would be ok]
The criterion in the cartoon,
Men in their 40s shouldn’t be skateboarding.
is of this third, or upper-bound, type, negatively formulated: men in their 40s or older shouldn’t be skateboarding. But then the cartoon:
Here, Brad totally screws things up, goofily taking in his forties in his doctor’s advice to convey an exact quantity — the decade of one’s 40s, not before and not after — so that Brad felt entitled to resume skateboarding when he turned 50
Brad’s idea is goofy because he’s disregarded why his doctor would have advised men in their 40s not to skateboard: because they’ve gotten too old for it, and it’s now become dangerous. All of that unsaid, because the doctor assumed Brad could figure it out.
Yes, that’s compressed, and there are lots of details, but the key idea is that stipulated criteria have reasons for being, and the nitty-gritty of these reasons governs when a criterion will be understood as lower-bounding, stipulating exactly, or upper-bounding.