Why cant u hear it?

ok, so you know how you can download this piercing high pitch sound as your ringtone for your cell phone. Why is it that over a certain age you cant hear it anymore? like over 18 or 21 or something, your not able to hear the high pitched sound...ppl do it in my cl and the teachers nvr catch em but the kids r always looking around the room and the teachers like wat r u doing? lol.