Audible frequencies and cell phones

Some of my co-workers were chatting at lunch yesterday about the highest audible frequencies that can be heard by people of various ages. Apparently, teens these days are programming their cell phones to use high frequency ring tones while in class. The frequencies are so high that adults can’t hear them, only teens can. One of my co-workers located a web site that has links to sounds at various frequencies. What’s the highest frequency you can hear? I got up to 18k using my headphones, but only 17k on my speakers. Anything higher than that makes me physically uncomfortable even though I can’t hear anything.