The most important thing is it should reflect how the characters talk. If everyone in a show (or R-rated movie) are swearing like drunken sailors, it paints a monolithic portrait. Not everyone speaks that way, or speaks that way in all moods and circumstances. Anything taboo that's overused loses its impact. That's why Kirk's "let's get the hell out of here" in City on the Edge of Forever seems to hold more import than all the swears combined in Seth MacFarlane's Ted, because it stands out. I think the bigger issue is the internet, especially uncensored forums like this one, are a big factor in "normalizing" profanity even with young kids. I mean, nobody seems to be aware or care in the least about the demographics of the audience of what they're saying because it's the internet and you can't physically see them. I'd say the #1 source of exposure of swears to kids are casual swearing like the above, on places kids hang out like Facebook or Youtube comments.