Most small audio devices and headphones don't reproduce 18kHz+ all that well, so it's not really worth worrying about too much.Also, human hearing loses acuity as we age.But yes, each frequency is the centre point for an adjustment that spans either side. So there's a benefit higher and lower in the frequency scale when adjusting 16kHz.
There's a good article here on the essential EQ frequencies and how they should be used: http://productionadvice.co.uk/using-eq/