As we age, we lose the extreme ends of the hearing spectrum because, unlike some other organs, 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. 

Almost everyone can hear 8,000 Hz. People under 50 years of age on average can hear 12,000 Hz. And 15,000 Hz can be heard by people under 40 (and people with better headphones than mine, at least I hope that's what it is). And so on. 

Here's the video from AsapSCIENCE that tests your hearing age:

Image: Caninest via Flickr CC BY 2.0