I “flew” in an Altitude Chamber years ago (when I was in my late 20’s) at Willie AFB. (When it was operational here in AZ.)
First, while I learned that I get a headache when hypoxic - and I never get headaches - I don’t know that that symptom would be enough in real life, flying a jet, to get my attention. And even in the rarefied setting of the Chamber (pun intended), where I wasn’t busy concentrating/distracted flying, even then I fumbled to put my mask on and flip the Regulator to Full Pressure.
Second, my airplane partner was the text book case of the person who doesn’t - or, more likely in her case, refuses to - recognize their symptom. (To be fair, there was another guy there - a weight lifter - who also did same.)
My airplane partner turned out to be the text book “happy drunk.” She just smiled at the Instructor as he yelled in her face to put her mask on. (Whereas the weight lifter reluctantly put his mask on.)
It was terrifying to see how easy it can be to crash as a chain-reaction side effect of hypoxia.
All this to say, while knowing your symptoms is better than not knowing your symptoms, I don’t know how much it prevents accidents.
Today we have inexpensive Oximeters and I’m a bit horrified to see that my O2 Sat at a mere 9000’ drops below 90%. (66 years old, in good health, no meds, slim, hike and exercise routinely. (It’s called "getting older.))
I think it best to know when you’re too old to be flying. Or at least too old to be soloing in something more challenging than a 152/172.
Or if you must fly, to always fly with another pilot.
Isn’t that what an older, wiser pilot would do?