Altitude sickness varies widely from person to person, and has little or nothing to do with fitness. One can acclimate (which is why in the Himalaya they spend time at the lower camps for a while).
Even climbing the 14ers in CA or CO, if one just stays overnight at 10,000 ft or so, it will help. Some people really suffer altitude sickness at as low as 5K ft, others (and I’m one of the lucky ones) never seem o be affected much other than a bit of light-headedness. If I were not already a bit light-headed, I’d probably not be climbing mountains all the time.
For fit hikers and climbers, effects are usually not noticeable until above 10K feet. If you are not prone to sickness, it’s just the lack of oxygen that causes you problems, and you just go slower.
There are two very severe types of altitude sickness, HACE (High Altitude Cerebral Edema) and HAPE (High Altitude Pulmonary Edema). The former gives you splitting headaches and can kill ya’ while the latter will make you cough your lungs out, almost, and you will actually drown in the fluid. That will kill too. Better to avoid these if you are prone to these.
Seriously, the ONLY cure for either of these conditions, if they start, to get the hell down as far and as fast as you can. The trouble is, the high altitude also affects your judgment, so many people start suffering these ill effects but don’t realize they should descend quickly. That is why you should always have at least one buddy if climbing at altitude.
The Air Force started studying this during WWII and there has been a vast amount of study of it since. One of the CA universities has a lab part way up 14K ft White Mountains in the High Sierra and another hut on top where they study it, and other studies go on in Europe and elsewhere.
The result of all this study to try to determine who will or won’t get altitude sickness is, oddly enough, I]nada.* They still have no idea why some people get it and others don’t. Highly fit marathoners can get it, and couch potatoes may not (providing they can get up there some easy way).
My wife begins getting headaches and dizziness at Flagstaff, AZ which is only 7,000 ft, while I can stagger all the way to 14,599 (highest have made yet, on Mt Whitney) without any sickness at all. In fact, I was 72 years old when I did that mountain, so neither age nor stupidity affects who gets it either, obviously.
Well, it does tend to keep the pussies off our mountains.