My son's experience with AMS:

 

My older son started attending the University of Utah in August 2004 which sits at an elevation of about 4000 ft. Prior to that he lived in Park City at 7000 ft. While in Park City , he never had any problems with altitudes in excess of 13000 ft. We often ventured to Colorado where the resort peaks are higher than Utah . When he visited us for Christmas, we decided to go skiing at the Snowbowl. He did fine with the elevation in Flagstaff . He was fine at the Snowbowl as well until we decided to try some of the more difficult mogul slopes. The extra exertion required on his part resulted in him developing a headache and nausea. We called it a day; drove back to Flagstaff and his symptoms gradually resolved that evening with hydration and rest. On our next trip to the Snowbowl several days later, we decided to do a short hike at 11,500 ft to find more challenging terrain and he again developed a headache but much milder with no nausea. The point I am making is that he seemed to adapt, but still developed the symptoms, both times related to exertion. Again the cause of AMS in this case was ascent to over 9000 ft, definitely overexertion and probably mild dehydration. Overexertion was the main factor in both cases. When you are an excellent skier as he is it is hard not to overexert as you want to try more difficult slopes. It's important to pace yourself.