From "Travellers' Health": "High Altitude", by Dr John Dickenson, Medical Superintendent of Patan Hospital, Kathmandu, Nepal:
Where does the risk of mountain sickness begin? This is a hard question to answer, because people vary so much in their susceptibility. The disease is well known in skiers at 8000 ft in the USA, and fatalities have occurred below 10,000 ft. Generally, however, in Nepal most of our patients have become ill between 12,000 and 14,000 ft...Speed of ascent is a greater risk factor than the absolute altitude reached...People below the age of 20 and above the age of 40 have been more severely affected than the 20-40 age group in some studies...There is no significant difference in risk between males and females.
(Concerning Malignant Acute Mountain Sickness)...One experienced Alpine guide, unused to Himalayan altitudes, was evacuated with malignant AMS in two successive seasons. Others have also had multiple attacks, but some seem never to be affected and we await a good explanation for these differences in susceptibility.