Everyone needs to read the August 2018 issue of Soaring magazine issue on hypoxia by Dr. Dan Johnson, Jean-Marie Clement, Patrick McLaughlin, & Dr. Heini Schaffner - the idea that anyone would not understand the effects of hypoxia and the cumulative effects of high altitude flying without O2 is scary and incredible. There is ample information available to anyone as to the folly of ignoring lower O2 levels. It will open your eyes as to the need to start O2 at lower altitudes regardless of age. Hypoxia is not an on/off physiological event - the effects are cumulative. And what one person can "get away with" does not mean another person can, or the same person can the next flight - the effects are individual. Incipient hypoxia stars at relatively low altitudes, and impairment at higher altitudes and/or prolonged exposure can be catastrophic. Listen to the recording cited in the August Soaring magazine issue "XC Tips" article by Garret Willat (now posted on the SSA website). PEEP or pursed lip breathing, as John F. points out, is a "now" expedient technique that will temporarily increase O2 levels. If you have to use it, and don't have supplemental O2, you must descend NOW while you still have the ability to do so. The basic recommendation from the Soaring magazine article is to start O2 at 5kft, regardless of fitness level, etc. The o2 is cheap compared to the alternative. And for those who regularly fly at high altitudes (10 kft), of course you have a pulse oximeter along to periodically check your O2 saturation level.