What is the 'Mosquito tone' used to deter teenagers?

The Mosquito is a device that emits a 17.4 kHz tone, which most people over about 25 cannot hear because high-frequency sensitivity declines with age. It went on sale in 2005 to disperse loitering teenagers and drew complaints as discriminatory. Teenagers soon turned the idea around, using the tone as a ringtone their teachers could not hear.

Source: Compound Security Systems (2005); UK Children's Commissioner 'Buzz Off' campaign (2008)