Face it, you don't hear as well as you used to. Remember that high pitched whine the TV set used to make? It hasn't gone away -- you're just not hearing it anymore. Age-related hearing loss, or presbycusis, is a fact of life: after age 18, the sensory cells inside the ear start to slowly die off. Each of these cells translates sound waves from the outside environment into electrical impulses that can be understood by the brain. But each cell doesn't react to every possible sound. Instead, they're specific -- each cell only reacts to a narrow band of pitches, determined by its position inside the inner ear. So when one of these receptor cells dies, the ear loses the ability to detect particular sounds. Typically, the high-pitched sounds go first.
Maureen McHugh found a site that lets you test the inevitable deterioration of your ears for yourself. It lets you test ultrasonic tones (as a prelude to downloading ultrasonic ringtones, natch), ranging from 8 kHz to 22.4 kHz. Typically, only people younger than 20 can hear the tones above 17 kHz. When I tried it out, I could hear up to 15.8 kHz without any problems, but at 16.7 kHz I had to turn the volume all the way up to hear a faint high-pitched noise. The program declared that my hearing indicated that I was "in my 20's," so I started feeling a little smug. Hearing as good as someone 10 years younger than me! Take that, time and decrepitude! Then I made my daughter try it out.
For her, the 16.7 kHz pitch that I could barely hear was an agonizing, annoying, loud whine. We kept raising the pitch. She heard them all until we got to the freakishly high 21.1 kHz pitch, which she described as "very soft and hard to hear." Ah, youth.