I'm totally ripping this off from Ace, but this is fascinating: the Noise Addicts Can You Hear This hearing test.
They say that most people over 25 can't hear tones over 15 kHz. Sho'nuff I pegged out at 15 kHz. My four-year-old was next to me while I was doing this and he heard all the way up to 19 kHz. I wonder if he's already suffered some hearing damage from me listening to loud music in the vehicle with him.
Certainly makes me think twice about turning up the volume in the future.
Also cool on the linked page, they link to a couple of products to help save your hearing - a set of earplugs that don't just block sound, but reduce the volume to a normal level; and some noise-blocking headphones so you don't turn up the volume so much.