I'm 38 and I still won't swear when my parents are around (maybe it's a cultural difference because I also can't imagine what a "they can say whatever they want in front of me" household would look like).
I was 30s before I could let myself swear in front of my parents. Who did I learn curse words from? My father, of course.
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.
My kids don't routinely swear in front of me. 10yo tries it out occasionally but then feels silly and stops. So our household is probably not too different from most when it comes to language use at home. But they don't get in trouble if they do, and I don't personally care - I try to emphasize the importance of thinking about what they're going to say to achieve their longer term goals. I'm more bothered by "you're stupid" than "fuck you", for instance, as the latter is more clearly just shorthand for "I'm angry at you". Though I try to get them to verbalize the latter if they can.
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.