The Tsimane are an indigenous people living in Bolivia near the Amazon rainforest, that still follow their traditional lifestyle, a combination of hunting, gathering, fishing, and subsistance farming. They are very active, and eat a healthy diet consisting of a large proportion of whole plant foods, about 15% animal products, and almost no “junk” (modern overly processed foods). They made the news a couple of years ago when they were found to have the lowest incidence of coronary artery disease of any population ever studied. Now it’s been found they have excellent brain health. Specifically, their brains don’t lose they volume with age as typical adults in modern societies do.
One of the study authors, neuroscientist Andrei Irimia from the University of Southern California, said “The Tsimane have provided us with an amazing natural experiment on the potentially detrimental effects of modern lifestyles on our health. These findings suggest that brain atrophy may be slowed substantially by the same lifestyle factors associated with very low risk of heart disease.” 
The good heart and brain health is despite the fact that, without access to modern medicine, the Tsimane have a relatively high rate of systemic inflammation caused by infectious disease. Those that make it to adulthood can expect to live to 70 years of age. Their longevity would probably be much greater if they could be protected from infectious disease.
1, Dockrill, P, “‘Amazing Natural Experiment’: In This Amazonian Tribe, Brains Don’t Age Like Ours”, Science Direct, 2021 (https://www.sciencealert.com/amazing-natural-experiment-in-this-amazonian-tribe-brains-don-t-shrink-like-our’Amazing Natural Experiment’: In This Amazonian Tribe, Brains Don’t Age Like Ours)