An average human when born can hear frequencies upto 25 KHz. However as he grows up, this ability decreases. Therefore teenagers can hear frequencies around 17KHz, whereas adults cannot. This is the theory behind the famous mosquito sound. Check what frequency can you hear..
http://www.zshare.net/audio/963330301a939b85/audiocheck.net_frequencycheckhigh_44100.wav Download this Media File – (Right Click)