And you don't need supplimental Oxygen at that altitude-I have backpacked for days at 12000 feet, with many excursions to 14000+ during that time. It is a common passtime...
The limit for aviation is much lower than for camping and mountain climbing. Makes a differance when you climb to altitude in minutes instead of hours! Your body doesn't have time to adjust, plus the relatively sudden lack of oxygen affects judgement as well.