> 10-12,000ft is generally accepted as negligible impact on cognitive function
That is just a guideline.
Each person responds to hypoxia differently and starts becoming hypoxic at different altitudes. One person might start having symptoms at 8,000 while another can function without oxygen at 14,000 for hours. It depends on your health and acclimation. All else equal if you live in Denver your tolerance to altitude will be greater.
This also applies to carbon monoxide exposure. For either condition you may not get headaches at all. Or you may get loss of color vision first. Or maybe your fingers tingle first. It is highly variable from person to person. Many people in those situations report they excluded hypoxia or carbon monoxide as a possibility because their symptoms didn't line up with the published lists, unaware that those lists are merely guidelines.
The one constant is that by the time you realize you are hypoxic or have CO poisoning you will either not care or be unable to fix the problem. This seems to be nearly 100% universal. Even when briefed ahead of time in a test scenario almost everyone will fail to take any action to rectify their situation on their own. Sometimes they can be coached into taking action (like descending or putting on an oxygen mask). For that reason a number of GA pilots use a pulse oximeter when flying above 8-10k to detect the signs early, while they are still aware enough to take action.
Also be aware that perversely CO causes your pulse oximeter reading to increase. Those chemical dots are useless, you need an actual detector and the sensor inside them is good for 10 years. Once bound to hemoglobin CO takes a long time (12-24 hours) to be removed and breathing oxygen only speeds that up by half. This is very unlike hypoxia which disappears in seconds once you start on oxygen. For aviation don't use household detectors - they have relatively high thresholds. On a long cross-country or multi-day flight you can accumulate exposure at levels low enough not to trigger a household alarm but cumulatively become very impaired due to the long half-life of CO in the body.
As someone with thousands of jumps from this altitude, I disagree. The VAST majority of 'first timers' I've taken on tandems don't feel much, if any, difference either going up.
Fair enough, I haven't jumped out of a plane, I was basing it on my climbing experience, which I realise now is not a valid comparison, I suspect that it's more noticeable in that context because of the high rate of respiration you tend to get when slogging it up big hills.
