But apparently I was wrong. It turns out, Robert Novak at Purdue University in Indiana (I think that's IUPUI) and other researchers have found that listening to headphones can worsen hearing loss. The researchers have been documenting a growing trend of noise-induced hearing loss, such as tinnitus, and found that it happens a lot with people who use headphones continuously.
I assure you, having worked jobs that require you to wear headphones up to 12 hours a day, that constant noise input takes its toll. My aural capacity is actually higher now, which means I can hear different pitches quite well, but my accuracy has gone to pot, which means I hear a muddled garble of whatever anyone says to me.
Lesson for all you parents out there: Tell your kids to keep the headphones off when possible.