Definitely, younger than 9. My friends in the neighborhood had older siblings, hence older friends and well, it's something of an enigma that I can keep up with the best of them.
I think swearing is something that once the novelty wears off, it can be tamed.

Which reminds me of this once incident with some "friends" growing up. I was young. And I don't recall what they did, we were down near a stream playing and I let off a mouthful. And this one girl, whom I'd had "problems" with for years decided she would race me to my home to "tell my parents" that I swore. (hmmm makes me wonder what went on in her home) but I owned it to my parents and they thanked my honesty. And that was it.