Understanding PRESBYCUSIS
Presbycusis, from Greek presbys “elder” + akousis “hearing”, is age-related hearing loss. Let's start with the hearing of your average 20-year-old.
This graph shows average sensitivity at each frequency for different age groups.
The dependent variable expresses sensitivity
This self-test is only an illustration; a proper test would also require a very quiet room and a calibrated computer-headphone combination.
The accumulation of loud noise takes a toll on your hearing, exacerbating presbycusis.
Studies of Easter Islanders and other peoples living away from industrialised society suggests that they suffer less hearing loss; unless they come live with us!
Maybe you should consider wearing earplugs the next time you go to a loud club or concert...
This video helps explain the biological effects of noise and age on your ears.

Some years ago, Mr. Howard Stapleton, of Barry, Wales, was annoyed by the young ruffians loitering outside a local corner shop. Stapleton knew something about hearing, and realized that playing a sufficiently high-frequency sound outside the shop would be maddening for the kids, but inaudible - or nearly inaudible - to older customers.
He tested various frequencies on himself and his children and decided on 17,600 Hz. Using the slider above, try it out.
Even with ears that are 19 years old (and I do mean old, as 19 is not young for the industrialised inner ear) you may be able to hear the sound, particularly if you crank up the volume (not recommended for extended intervals!). As you get older, the SPL you require to hear this frequency steadily increases- review the hearing loss and age graph at top.
Some ingenious schoolkids have turned this idea on its head and there are now ringtones available that play at 17,000 Hz - the better to use your mobile phone at school without your teacher knowing!

Differences in frequency sensitivity are also exploited in other technologies. Dogs are sensitive to frequencies much higher than humans are. The sensitivity of various species normalized so that threshold at the species' best frequency is 0 is plotted at the right, from Dooling et al. A great tit is a small songbird, with an inner ear structure similar to that of dinosaurs.
Brachiosauruses were enormous. Their inner ears were very large, allowing them to hear ultra-low frequencies.

Dogs are better at ultra-high frequencies. Some dog whistles emit frequencies within the range of canines' ears, but too high for humans.

So you can get your dog's attention (or annoy him, at the very least) without hurting your own ears.