Duh. Any intelligent kid can figure out other people say these kinds of words.
If you think they should be taught to *not* say them, how are you any different from the rest of us "uptight" people?
If you are offended by these words or don't want your kids to hear them, then lock them in a room. People swear. That is reality. If *you* don't want your kids to swear, or want them to hear it- you control that until they're old enough to make their own choice. But you simply can not control what other people do or say.