Two schools and a stadium in Auckland have been fitted with an alarm which emits an annoying sound that only people under age 25 can hear. The Mosquito Anti-Vandal System has been termed illegal by many human rights groups since it specifically signals out young people as potential criminals. The alarm is set at 17.4 kHz which can generally only be heard by people under the age of 25. It has been described as similar to dragging fingernails across a blackboard. The goal of the system is to prevent graffiti which has been common in the stadium. An under 25 year old reporter described how he was unable to tolerate the noise while his forty year old companion heard nothing.
I guess the next process to be developed is for the aged and it can be set to prevent old foggies from getting food out of the refrigerator at night when they should be sound asleep.