My sister and oldest daughter are over 13, and my youngest is nearly 13. I realize that they hear profanity. I also realize that there are themes too strong for underage children in this game. The people that I would recommend this game to aren't under 13.
Honestly, I'm only really offended by "godsdamned" (this is due to my religion). I know that I can just quit playing, but this game is incredible. I like to share the experience of a great game with the people that I talk to frequently (most of whom share my religion).
I agree with Demme that a filter for NPC dialogue could probably be coded the same as the PC language filter. If this is not the case, then I understand the dev's decision to decline this request. Let them decide, though, instead of making blind assumptions.
I'm going to create a dedicated thread about the profanity filter soon. We've muddied up the OP's post, so I'd like to delete these from here after my thread is up.