They're also incredibly active compared to any "civilised" people. There's a lot of evidence that sedentary lifestyles are bad for bodies.
Presumably they're also, on average, younger than "civilised" peoples. Life expectancy for an Amazonian Tribe was just 53 years. https://www.sbs.com.au/food/article/how-to-eat-like-an-amazo...
Muscle and joint issues are a symptom of aging. If nobody's making it to 60, then aging really isn't the same issue.