Comment by daemonologist

Potentially not even so high as that. You often see 2,500 m (8,500 ft) used as the beginning of "high altitude" - at that point some studies have detected cognitive effects from long-term exposure. (10.1007/s11065-004-8159-4) A lot of research is focused more on ~4,000+ meters though due to the existence of the Tibetan Plateau.