As we age,
the inner ear doesn't regenerate. Nerve cells called "hair cells" pick up different frequencies and send them to the brain for processing. The hair cells tuned to high pitches are the first to encounter sound waves, so they experience more stress.
Just about everyone can hear 8,000 Hz and
for people under 50 years of age can hear on average
12,000 Hz. 15,000 Hz can be heard by people under 40.