Supposedly as you age your hearing starts to go. Here is a selection of tones from 11 KHz to 18 KHz. The top end (18 KHz) is the end that drops off as
you age. This is the end that has been used to drive teenagers away from malls.
So visit the site and check out just how well your hearing has aged.
Personally I think its a hoax, there is nothing being played above 14KHz as far as I can tell.