I can pretty distinctly hear 18kHz sine waves and so can many of my friends, I don't think it's that rare. If you actually weren't able to hear past 16kHz you

I can pretty distinctly hear 18kHz sine waves and so can many of my friends, I don't think it's that rare. If you actually weren't able to hear past 16kHz you wouldn't be able to hear the ringing caused by a good filter above 16kHz, and you wouldn't be able to tell a difference in the transients either.