I think it would be a very unusual situation indeed for a 13/14 year old to never have heard the word fuck.
I personally believe that we speak a number of varients of language depending on the audience we have. This also, to me, explains the nuances of why you can tell one person's written language from another's. It also explains why we write a business letter using different vocabulary and sentence structures as we would do in a text to a friend or a post on Mumsnet.
For this reason, we can see why it's unusual to use swears in front of young children, or in front of other people's children. Or in front of elderly relatives or a priest. People might, but then they'd be using another form of language entirely; a language chosen to intimidate and shock. That's not based on the individual words, but how they are used.
At 13/14, children are have already learnt the different forms that they might use to their teachers or their parents or their friends' parents. You can see Kevin and Perry as a supreme example of this use of diverse languages depending on audience.
I personally swear a lot in my usual, day to day, language. Now my son is 11, I expect him to swear freely, but I also expect him to be able to identify and use these different forms of language correctly. I do not expect him (and would be angry with him) to swear in front of teachers or his elderly and loving grandparents. I expect him to use the appropriate form of language for certain situations. I personally do not ban specific words to outside of any acceptable form of language. I'm not sure what that would teach him.
I do expect him, as in, I am not surprised by, to swear in front of his friends. I also expect him (not surprised by) him to swear in front of me. To me, it is a vital part of understanding where I am, at that moment, to him.