http://hosted.ap.org/dynamic/stories/T/TEEN_BE_GONE
The high-frequency sound has been likened to fingernails dragged across a chalkboard or a pesky mosquito buzzing in your ear. It can be heard by most people in their teens and early 20s who still have sensitive hair cells in their inner ears. Whether you can hear the noise depends on how much your hearing has deteriorated...
... [T]he noise can be heard by animals and babies, but is bothersome only to children older than 12 and becomes unbearable after several minutes, making it a perfect teen-repellent. The same sound is also used as a cell phone ring tone meant to fall on the deaf ears of adults, and is a popular download on the Internet.