It's not necessarily geared toward teens and adults; it's meant for all ages. The homepage clearly states that "Snap*!* is... for kids and adults." Although teens and adults commonly use it, younger children use it, too.
This is a perennial issue. Most of our users are > 13, but we do get the occasional eight-year-old. Our first version of the ToS said we're aiming at PG-13, and I still think that's the best way to serve our core user base. But I really really don't want to have a fight with some kid's angry parent, or just lose the kid because their parent won't let them on here any more.
We could try for a technical solution in which we remember everyone's age and let authors attach a rating to their project. But, life is too short for that.
The following is not Snap! policy, just a personal opinion! I believe that people should not be afraid of words, and kids in particular should be taught not to be afraid of words, not even that word. Using a word to attack someone is just like using a baseball bat to attack someone; it's the attack that's wrong, not the bat, or the word. (I don't take this argument to the point of stupidity; there's no reason why anyone needs an AR-15 in their house, other than to attack people.) It drives me crazy to have to split hairs over what words you're allowed to use. Life's too short for that, too.
I suppose labelling the project's thumbnail and/or start screen as PG-13, or R, or whatever is a plausible solution.
I also am more inclined to defend a project that tells a good story in which strong language is important to telling it, than a project that just throws four-letter words around to show off. And we definitely don't want any X-rated (pornographic) stuff on the site. Insert here a discussion of why it's okay to portray violence, including some pretty horrible violence, to children, but not okay to portray sex.
Homework: Go to a public library in a pretty big city, not your school library, and look up Dick Gregory's autobiography. (If the name doesn't mean anything to you, you might have to look up some of his comedy albums first.)
I wasn't joking, but I wasn't expressing my own opinion either. People get beaten up on TV and in the movies all the time, and if it's a gangster movie they get beaten up severely. You're allowed to make projects, even in Scratch, in which you shoot at NPCs and when you hit them they disappear in a cloud of red.
What's important isn't how realistic it looks. (Well, that's important too, for other reasons.) What's important is that even animated violence sends the message that violence is okay. (I don't think that about over-the-top cartoon violence such as steamrollers and falling anvils. But shooting games, even stylized ones such as Space Invaders, yes. Tons of those on Scratch, and here too. Why is that okay? I'd rather have you use dirty words than that. YMMV.)
Edit: No, let me be more precise. I'd rather we make it okay for you to use dirty words than make it okay for you to depict violence. And really I'd prefer not to rule anything out, so that we are not seen to be preferring some things to others.