Clicking the play button below will produce a tone that is generally only heard by people under the age of 25. It has been used as a deterrent device to keep teenagers from loitering in malls and shops, and sounds similar to a buzzing mosquito. Typically the longer you listen to it, the more annoying it gets.
Try it and let me know. I can hear it, at 46 yrs old with a combined hardware and software hearing loss of 40%.