Science fiction can't make this stuff up: http://www.buzzfeed.com/buzz/Teenager_Deterrent.

From the company website:

The Mosquito™ ultrasonic teenage deterrent is the solution to the eternal problem of unwanted gatherings of youths and teenagers in shopping malls, around shops and anywhere else they are causing problems. The presence of these teenagers discourages genuine shoppers and customers’ from coming into your shop, affecting your turnover and profits. Anti social behavior has become the biggest threat to private property over the last decade and there has been no effective deterrent until now.

The scientific explanation:

It seems that there is a very real medical phenomenon known as presbycusis or age related hearing loss which, according to The Merck Manual of Diagnosis and Therapy, "begins after the age of 20 but is usually significant only in persons over 65". It first affects the highest frequencies (18 to 20 kHz) notably in those who have turned 20 years of age". It is possible to generate a high frequency sound that is audible only to teenagers.

The sales pitch:

Is your business suffering from anti social youths driving your customers away or generally causing damage or nuicance?

Are you bothered by crowds of teenager’s hanging around your street and making life unpleasant?

Mosquito™ ultrasonic deterrent can solve your problem.

Purchase Mosquito™ now!

My company would have succeeded if it wasn't for those pesky kids!

This is a British company, but stay tuned for my legal analysis should this product cross the pond.