A diet containing high polyunsaturated fatty acids protects the cognitive abilities of people over 60. Specifically omega-3 fatty acids have particularly strong protective effects. Epidemiologists from Qingdao University have reported this.

The study
Data used from 2,496 people over 60, who participated in the US National Health and Nutrition Examination Survey (NHANES) between 2011 and 2014. The study participants had told what their diet looked like. It was estimated whether the cognitive abilities of the elderly are compromised by a series of tests conducted by psychiatrists and gerontologists.

The study participants were divided into 4 equally sized groups. Next, the effect of the intake of omega-3 and omega-6 fatty acids on the probability of being in the group with the lowest score was calcualted.

Results
The more omega-3 and omega-6 fatty acids were consumed by participants, the less likely they were to score low on the cognitive tests. Expressed in milligrams of fatty acid, the impact of the omega-3 fatty acids was particularly significant. If you consume 1-2 milligrams of omega-3 fatty acid per kilocalorie intake per day, you're good. 

Mechanism
In neuroscientific literature, it was found how omega-3 fatty acids might keep brain cells healthy as the years climb. Omega-3 fatty acids are important for the functioning of neurotransmitters and also make cells more vital and robust. In addition, omega-3 fatty acids inhibit inflammatory processes that can affect brain tissue during aging processes.
 

Source
Nutr J. 2020 Mar 28;19(1):25.