It's not usually everybody... Even at +11,000' or so. If we were all born in Buffalo, that's around +500-600' msl. It is a big jump
Back in Scouts, our Philmont crew, we had one Scout feeling the effects of high country. Staying hydrated is key. Eating well too. We got back down below 10,000 and he squared it away. Which was a good thing... Would have sucked to hike him out. This was in mid-1980s before modern communicative technology.