Eric Greenwell wrote:

On a hot soaring day
at Minden, or elsewhere in the high desert ares, density altitudes can
easily be 8000' or higher.

For breathing, density altitude doesn't matter. It's the pressure
altitude, or more exactly, the partial pressure of the oxygen, that matters.

Nasal cannulas are comfortable enough, and
the newer oxygen systems economical enough (especially the EDS
system), that I'd say there is little reason for "most people" to
wait until 12,000'

Absolutely.

Here in Switzerland, there's no legal requirement to use oxygen for
private pilots. However, there is one for commercial pilots. It roughly
sais: Use oxygen when you're
- above 13'000 ft or
- above 10'000 ft for longer than half an hour.

It's not enough to just stay awake, you must stay alert.

Besides, many pilots report less headache the day after the flight when
they use oxygen above 10'000 ft.

Stefan