I have spent many days at
elevations in excess of 15,000 feet, several times over 17,000 feet,
including a climb to an 18,300 foot peak.

Charles,

 

Really!  You were young (less than 60), in good shape, not too overweight, etc.

 

Some of us old farts - even though we've spent weeks skiing at high altitudes - are just not "sharp" enough to do without supplemental O2 as we climb above the third floor....