Who actually hears 20 kHz
The 20 Hz-20 kHz range quoted everywhere describes a healthy child. Infants genuinely approach 20 kHz; teenagers commonly reach 17-18 kHz (the basis of the mosquito tone loitering deterrent); by the mid-20s the ceiling for most people has dropped below 17 kHz, and it keeps descending a few hundred hertz per year. An adult who hears a genuine 20 kHz fundamental is vanishingly rare.
There is also an equipment asterisk: consumer audio is engineered to 20 kHz as a spec ceiling, and many speakers, amps, and codecs behave unpredictably at exactly the edge - some tweeters barely reach it, and any distortion in the chain produces audible lower-frequency artifacts that masquerade as "hearing" the tone. If you think you hear this page, you may be hearing your equipment's imperfection rather than 20 kHz itself.
Beyond the human window
Plenty of ears keep going where ours stop. Dogs reach roughly 45 kHz (the principle behind silent dog whistles at 23-54 kHz), cats about 64 kHz, and bats and dolphins echolocate at 100 kHz and beyond. Human ultrasound applications start right above this page: cleaning baths at 20-40 kHz, medical imaging in the megahertz.
Audio formats above 20 kHz (hi-res sampling at 96 or 192 kHz) remain debated precisely because controlled listening tests struggle to show adults distinguishing content above the limit - the engineering arguments for higher rates are about filter design and processing headroom, not about hearing 30 kHz. Where your personal window actually ends is measurable in two minutes with the hearing age test.