the NY has an interesting story about a british inventor who created an device that produce a high frequency sound  that can be eared only by people younger than 20, and is a prefect repellent.

the Sarkozy symphony ;-)

The device, called the Mosquito ("It's small and annoying," Mr. Stapleton said), emits a high-frequency pulsing sound that, he says, can be heard by mostpeople younger than 20 and almost no one older than 30. The sound is designed to so irritate young people that after several minutes, they cannot stand it and go away.

 Article

Are we going to an anti-youth society ?