According to Scientific American, kids can hear sounds above about 16 Hz, which are difficult or impossible for adults to hear. As humans age, presbycusis or age related hearing frequency loss begins, starting at around age 18 in most individuals and continuing throughout life.
As BBC News explains, this phenomenon is something that security contractors have tried to take advantage of in a practical sense. The "Mosquito" is a device designed to deter youth crime in subways and other public areas. Initially released in 2005, the Mosquito anti-loitering device emits a loud, high pitched audio pulse, only audible to those under the age of 25.