When I was a lot younger, I would routinely fly gliders to 10,000’ with short periods up to 12,500’, with no hypoxia symptoms. Note that I lived then and now at 3500’ ASL.
Two years ago, aged 73 but still with a Category 1 medical, I spent 10 days flying at a gliding camp at Valemount BC (2600’ ASL). At the end of the camp, I ferried a tow plane through the Rockies, back to its base near Calgary. It was a spectacular CAVOK day with booming thermals, which made for a rough trip, but I thoroughly enjoyed it and took lots of spectacular photos.
The first two hours of the flight were between 9,000’ and 10,00’, as I followed the Jasper-Banff Highway (the Icefields Parkway). At Lake Louise, I took the direct route home “over the rocks” and flew the last hour at 10,000’ to 11,000’.
After I had landed and shut down, I was lethargic and slightly disoriented. Although I could recall many details of the flight, including a perfectly satisfactory landing, I couldn’t remember if I had put the flaps down!
I realized I was hypoxic. Now when I fly gliders on thermalling flights (8,000’ to 12,000’), I use oxygen which not only avoids hypoxia, but has the added benefit of reducing tiredness at the end of a long flight.
The Mountain High electronic regular that I use, feeds puffs of oxygen during inhalation, through a nasal cannula. It is so convenient and easy to use that I use it for the whole flight. Hopefully no more hypoxia for me.