In the April 2016 Journals of Gerontology a large study reporting over a thousand older adults with hearing impairment was published. The study looked at the impacts of moderate-to-severe hearing loss versus mild loss on memory and cognitive decline. The findings were remarkable for there being a 55% higher rate of decline in the more severe hearing loss patients (OR 1.10-2.19 @ 95% CI).
It is of course uncertain why hearing impairment causes cognitive decline but professionals in the field cite the intuitive relationship of reduced social engagement and reduced ongoing mental stimulation as being leading reasons. It is surprising for most people to realize just how disengaged a person can become when they are constantly frustrated with communication hurdles.
Treating hearing loss may be more important than diet or medication in helping to improve cognition late in life. A good physician or gerontology evaluation is the first step. Seeking out an audiologist or a physician-audiologist team is the best bet for properly assessing the severity of loss and making the best most meaningful recommendations for improvement.