Hilariously enough, this is my field of expertise. I think this may be the first "sound" related thread on MHF that I personally didn't start.
Anyway....NOBODY should be purposely pumping 18KHz or more into their eardrums. So, if you care about your hearing, don't even bother trying to listen to those frequencies.
YES, dogs and other animals will become EXTREMELY annoyed with you if you continue to make these frequencies audible. Imagine if I taped your head to Metallica's PA horns during the middle of an obnoxious solo...you wouldn't appreciate that very much, so please be nice to the little guys.
As we get older, we DO gradually lose our hearing, but lets not get the ridiculous idea that there is some sort of "secret" ringtone that adults can't hear. Some kids may have fooled a senior citizen, but it's not as the reports make it seem. Even at frequencies you cannot hear, most people will still feel an uncomfortable vibration. To make it seem like these kids have defied the laws of physics and sound is just hilarious.