13,000. Hypoxia? Most people cannot handle 13,000 but for a few minutes. Besides, O2 is required at 12,500 for anything over 30 minutes.
I've been to the summit of Pikes Peak at 14,114 feet multiple times. Folks were not jumping rope, but nobody was keeling over.
If you're in the area, try the cogwheel railroad from Manitou Springs.
Sorry, but that can’t be right. I’ve spent over an hour at the top of Pikes Peak > 14,000’ and although I would get a bit dizzy if I over exerted myself, otherwise was fine.