Human high-frequency hearing declines markedly already in the twenties

Evidence

The nominal range of human hearing is usually quoted as being from 20 Hz to 20 kHz, but during a person’s twenties, the upper-frequency region reduces greatly in the range above about 16 kHz. The mosquito unit exploits this by emitting a sound between 16 and 19 kHz, which can only be heard by those less than 20 to 25 years of age. The usual range over which the mosquito operates as a deterrent is around 15 to 25 m. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2573

Categorization

• XREF: Connects to the 20 Hz–20 kHz nominal hearing range likely given earlier in the book; the practical ceiling is far lower.
• SHARE: Nerdy specific: the 16–19 kHz window that makes the device work only on under-25s — send to someone into audio gear.