A few weeks ago, I helped a colleague chaperone a field trip. Although it was a good chance for me to get out of the office and see some of my former students, the teacher really didn't need an additional chaperone. He had 17 charges---most of them high school seniors with an eye on academic pursuits. They were the kinds of kids who would have been just fine without any adults to shepherd them. And yet, they were still kids.
The students had a bit of an extended break at lunch and a few of them were chatting a bit noisily in the cavernous lobby of the science institute we were visiting. They weren't being "bad" or inappropriate, just momentarily self-absorbed in the way that teens sometime can be. I wandered over and whispered a reminder to them to use their "indoor voices" and smiled. They giggled and understood. Their teacher asked what I had told them and after I passed along the information, he said, "I didn't think that would work. It doesn't work on my nine-year old anymore."
That's the funny thing about knowing your audience. You can use the "indoor voice" or "six inch voice" phrase with little ones or with late teens and they're understanding (the older ones are amused at the reference). Junior high kids? Not so much. They're a bit too self-conscious, and want to distance themselves from child-like reminders. Their brains don't know how to read the facial expressions of others well---they can't always tell if their teachers are joking. Feelings are hurt very easily. It would be a tremendous insult to a junior high aged student to tell him or her to use an "indoor voice."
I think that one has to be an ethologist of sorts in the classroom. You have to live within the culture of the age group you're teaching. It's not always easy because a teacher has so much other professional and personal life experience to bring to the table. There has to be some trial and error in learning which boundaries you can push. You find some tricks to use with each grade level and age and then learn to adapt them with others. The audience for all of this will be active in shaping things along the way. They want you to know them.