I think the first time my son said the 'f' word, he got the tone and the body language exactly right for the context in which he used it (a frustrated/resigned "fuck's sake", with roll-eyes and throw-head-back, look up to the sky). He might have been five years old at the time. And back then, I really didn't say it around them very much (out of an attempt to be somewhat responsible).

They're absorbing everything. They're scraping the entirety of your personality to build their own.