With age, often, comes hearing loss. There are two main reasons for this. First, as a person ages, everything becomes less flexible. That includes the eardrum, a membrane that works by vibrating in response to sound—as the eardrum becomes stiffer and less able to vibrate, it becomes less able to detect sound and transmit it to the brain.
The other reason is a life of noises. Loud noise damages the ears. The damage may be minor, but it is also lasting, and it accumulates. After a lifetime of listening to loud noises, a person's hearing will start to deteriorate. In fact, more and more people are being diagnosed with hearing loss at younger ages than before. This is partly due to more sensitive and reliable tests, which people are undergoing earlier in life, but it's also because of the loudness of modern living, particularly headphones.
Often, however, hearing loss in teenagers is missed. Because it is thought of as a condition, if not fate, or old people, hearing loss is not always even suspected in young people, despite more than half of all people with some form of hearing difficulty having developed it in childhood. Compounding this, many adolescents have difficulty noticing or recognizing hearing loss and adequately conveying that to medical professionals. In principle, young patients will be asked questions designed to find those who may need more objective audiological testing. However, the questions asked are not always useful for patients in that age range, leading to underdiagnosis.
Moreover, noise-induced hearing loss can start as young as age 20. This occurs when the "hair cells" within the ear are damaged by exposure to loud sounds. The hair cells, once destroyed, do not recover, leading to a permanent reduction in auditory acuity. This doesn't merely damage the ability to hear, it actually changes the way the brain processes speech. Not only do sounds not get through as efficiently, when they do get through, they are not processed correctly. However, new medical techniques may hold out hope for recovering hearing ability lost to noise. Researchers have developed a clear picture of the structure of the cells supporting these hair cells, and had some success in repairing them in experimental animals.