I'm 43 and can still hear the annoying mosquito teenager deterrents, maybe not as loud as a teenager can, but they are still really annoying. I also made a device that emits 15625Hz from one tweeter and the same +/- a few Hz from another and it seems most people can hear that - It was designed to be annoying, go somewhere where people are rowdy or speaking with raised voices and after switching on everything goes eerily quiet - no-one can determine the location it's coming from due to the frequency being high and the (inaudible)  beat frequency has a weird effect on people.