Recent studies have found a link between hearing loss and dementia. People with a mild loss are twice as likely to develop dementia as their peers with normal hearing and people with a severe hearing loss are FIVE times more likely to develop dementia. You can read a short summary article about this study here:

http://www.aarp.org/health/medical-research/info-02-2011/hearing_loss_linked_to_dementia.html

More recent studies seem to reveal that wearing hearing aids help prevent this from happening but it's early in the research and nothing is definitive yet.