I went to Catholic school through fifth grade, so I didn't even really hear any foul language until I transferred to public school in sixth grade. I brought home some interesting new vocabulary words...
I think the first time I said anything that could be considered swearing, was when I was about 6. My dad was yelling at me about something (he was always yelling at me about something) and he said, "Up your nose!" So I said, "Up yours!" - honestly not knowing that that generally referred to another part of the anatomy.
I couldn't sit down for a week after that.