E.D.
My son is 6 and he still does this, although he seems to be getting out of it. At age 4-5 it was the worst, he was just facsinated with potty talk. We would threaten to wash his mouth out with soap, and one day my husband actually did it. Even this didn't stop him for very long. I could not just ignore it either, I just feel it's not right. So I always tell him to stop, it's unacceptable. I found one thing that worked: the other morning, I was lying in bed and I heard my two boys in their room talking. Suddenly, I heard my 6 year old say the word bu**crack, and then his little brother repeated it. I jumped up and went in there, and took away one of his favorite toys, stashed it in my closet. I told him he'd lose one every time he used bad language. I haven't heard any since!
And don't worry, you're not the only one experiencing this. My son's friends were all doing it too!