Headphone boxes advertise frequency response to 40 kHz. Your ears stopped going that high a long time ago. Enter your age for a rough idea of where your own ceiling sits — and how much of the spec sheet is being sold to a part of you that has already retired.
It does not mean high-resolution audio is pointless, and it does not mean an expensive headphone is wasted on you. Almost everything that makes headphones sound different from each other — tonal balance, bass extension, how the midrange is voiced, distortion, how the drivers behave under load — happens far below 10 kHz, comfortably inside anyone's range. A 45-year-old with a 14 kHz ceiling hears essentially all of what separates a good headphone from a bad one.
What it does mean is that ultrasonic frequency-response claims are marketing, not a feature. When a spec sheet leads with "5 Hz – 40 kHz", it is advertising territory nobody's ears occupy, and it tells you nothing about how the headphone actually sounds in the range you live in. Judge by tonal balance and measurements in the audible band instead.
This is an estimate from population averages, not a hearing test. If something has changed recently in one ear, or you have ringing that will not settle, see an audiologist rather than a calculator.