My brother's rule for his kids regarding swearing and other NSFW words is: "You'll probably hear adults use these words from time to time, but you can't repeat these words until you start paying taxes."
I tend to favour my friends’ policy about swearing with their kids: everybody feels differently about it, so you have to be careful about swearing around other people until you know whether it’s okay with them. At home? Probably fine. At school? Probably need to be more careful about it.
It’s much more realistic to teach nuance than to outright forbid the behavior imo.
( Although I suppose we should probably acknowledge - 1) these are not our kids, and 2) every kid is different. Some kids appreciate the value of choosing your words to fit your audience, some adults never learned. )
My point is this isn't even a bad thing. As long as they learn to not swear around certain people (like dad), then that's what you want
Dad (probably) doesn't care, dad just doesn't want to be called to school by teacher when his kid repeats few swear-words.
I've been called by a school about my kids' swearing. I told them I don't see what the fucking problem is.
As a former child myself, that's exactly how it goes; the purpose of the exercise is not to forbid them to swear, it's to learn when it's appropriate.
This was more of an individual thing than cultural though. My family is stereotypical North American Protestant, of Northern European descent. And neither of my siblings showed the same restraint in using "foul" language around my parents that I did.
swear words are exclamatories, meant to indicate extreme emotion. so using a lot of swear words is like shouting all the time, or using all caps all the time in writing. it washes away the richness of expression and makes the swearer seem unable to properly contextualize and express their emotions. our brains are attuned to wash out sameness and pick out differences, especially sharp ones. that's also why selective swearing is very effective, and constant swearing is ineffective.
there's certainly much more to it than just that, but that should be the core contextualization of swearing for kids. let them experiment on their own but just like shouting all the time, let them know it won't be tolerated in most situations until they can effectively and appropriately wield this grammatical tool. don't impose some indirect, off-the-cuff age cut-off, which they'll surely understand as being arbitrary and therefore easily (and rightfully) dismissed.
My house, my rules?