I didn't downvote, but I'll make a guess:

Native Americans comprise a broad range of cultures that developed in a variety of geographies and climates. Making sweeping generalizations about their traditional lifestyles is both flagrantly poor anthropology and a known practice among peddlers of diet pseudoscience.

My guess is it is impossible to say, mostly since this requires lots of older people and the main cause of death in those cases (as in non modern medicine cases) is generally not old-age related. Might also have to do with the limited set of data available.