"Wait, did you hear that?" This is a question that you might hear a teenager asking an adult regarding a high frequency noise nicknamed "teen buzz", and the answer you will likely hear from the adult is "No." Teens have been using this high pitched noise as their cell phone ringtones during school because the teachers cannot hear it, and the science behind this is a degenerative condition called Presbycusis. It affects most of the adult population, and in a nut shell, it impairs your ability to hear high frequency noises (greater than 17khz) starting around age 18 and progressing as you get older.
I find this really interesting and would like to know more about the true reasons behind this phenomenon. What's even more interesting is that some adults can hear the noise, which suggests that there might be some aspect of the brain that is different in the adults that can hear it than the ones that cannot.
A personal example of this occurred in my high school Accounting class, when one of the kids was playing the ringtone over and over as a prank because he thought the teacher couldn't hear it (like most adults can't). However, after playing it about 5 times, the teacher called the kid out and told him that she heard it all along and made him look like a fool in front of everyone. If I were the teacher, I think that I would be more excited about the fact that I could actually hear the noise than upset about trying to be pranked by a student!