From my experience they learn from the older kids as soon as they start school. At least in my culture. My daughter came home saying "shit what the fuck" (in English, despite English not being the native tongue of the locality) one day. And now she will curse like a pirate when she hurts herself. But she already knows they are words that people attach special meaning to: she avoids them when "not needed", for example. Except when she wants a reaction, which she never gets from me at least.
We don't use many such strong words ourselves, but like with many things she learned on her own in the cultural melting pot that is public school.
We don't use many such strong words ourselves, but like with many things she learned on her own in the cultural melting pot that is public school.