The Sunday Mirror ran an article about a device, called the Mosquito, that promises to allow shopkeepers to get rid of the crowds of surly youths who like to congregate outside their shops. The article states:

The machine, which is hidden within the lights of corner shops, uses ear-splitting ultrasonic soundwaves. It is being hailed as the answer to clear away underage drinkers and vandals from the doorways of late-opening stores. The 9in-high device - called the Mosquito - has a range of 20 to 30 metres and emits a piercing sound only clearly audible to under-20s. The sound is said to be "extremely unpleasant", but not harmful.

The website of Compound Security Systems, maker of the Mosquito, further explains:

Mosquito is essentially a sounder unit that emits a very high (ultra-sonic) tone that is completely harmless even with long term use... Research has shown that the majority of people over the age of 25, have lost the ability to hear at this frequency range... The longer someone is exposed to the sound, the more annoying it becomes. Field trials have shown that teenagers are acutely aware of the Mosquito and usually move away from the area within just a couple of minutes. The field trails also suggest that after several uses, the groups of children / teenagers tend not to loiter in the areas covered by the Mosquito, even when it is not turned on.

I'm not sure about the science here, but it does seem plausible to me that younger people would be able to hear high-pitched sounds more easily than older people. If this does work, I would definitely consider installing it to annoy my college-age neighbors who enjoy playing basketball in their backyard at midnight. (Thanks to Eric for the link.)