Does Peter know what periods are?

I don't think anyone has được trao him the talk, and honestly if someone did I think he wouldn't talk to them anymore. That's way too grown up for him and if brought up he thinks it is disgusting.

Yes, I totally agree. Even I cant stand talking about it >-<. Since this is a thing that only is personal with girls (and not boys) he must not understand what it is and I bet he doesn't want to find out either XD

He's MUCH too innocent to know about that stuff, and I agree with OneMoreBeliever that he'd be disgusted and (most likely) very angry at whoever would've told him.