17,500'??? Remember being at 14,000 on Pikes Peak years ago in youth. Could hardly breathe as air (oxygen) so thin. How's this guy talking/breathing at 17,500?
It depends on what you are doing. I used to live at 8,500ft and our runway was 8,200ft and I was a flight instructor and daily spent time at around 12,000ft plus with no problems. It's the flat landers that have the problem at altitude. As for flying in an open environment at such altitude, you get a sort of ram air effect that helps you get more oxygen into your lungs when flying in an open cockpit.
There is also a technique that you can also apply when flying higher with no oxygen, to help. You sort of take gulps of air and then compress that in your lungs, by sort of straining. By doing that you get more oxygen into your system than if you just breathed normally.
I once took off at 8,200ft and took my open cockpit plane up to just over 20,000ft by about a hundred and a bit, feet, before heading right back down. Yes, it was a bit difficult to breath but by forcing the lungs like I explained, I had no problem. Naturally you don't want to stay up at that altitude with no oxygen, but it is doable for a short period. Of course that would not be doable for someone who lived at or took off at sea level. When I lived just outside of Breckenridge, I was living at just over 10,000ft and we used to consider Denver as sea level, for us!
Another strange thing is that skiing at 11-12,000ft does not seem to be too much of a problem, but just bending over to do up your boots or climbing a few stairs at 11,000,ft seems to take all the wind right out of you.