A significant percentage of high altitude mountaineers including a number of Everest attemptees, when tested under MRI, showed evidence of brain damage due to hypoxia or lack of oxygen. Therefore, it makes sense to acclimate to higher altitudes and above all, to minimize your exposure to high elevations above 14,000 feet.
Does this mean that you should avoid mountaineering? It is best to exercise caution - minimizing the level of exposure. Read more at the Well Blog from the NY Times. The benefits of this kind of exercise in terms of cardiovascular fitness may reduce overall health risk, so this is another factor to keep in mind. Try the Everest test.