The pleasures of trekking in the world's highest mountain ranges cannot be overstated. Neither can the dangers. Altitude sickness can occur in some people as low as 8,000 feet, but serious symptoms do not usually occur until over 12,000 feet. Even then it is not the height that is important, rather the speed in which you ascended to that altitude.

Acute mountain sickness (AMS) is actually more common in fit young men because they are more likely to attempt a rapid ascent by racing up the mountain like some indestructible super hero! As a general rule, it is far safer (and more enjoyable) to avoid altitude sickness by planning a sensible itinerary that allows for gradual acclimatization to altitude as you ascend, (you can race back down as fast as you like!).

Acute Mountain Sickness (AMS)
AMS is very common at high altitude. At over 3,000 metres (10,000 feet) 75% of people will have mild symptoms. The occurrence of AMS is dependent upon the elevation, the rate of ascent, and individual susceptibility. Many people will experience mild AMS during the acclimatization process. The symptoms usually start 12 to 24 hours after arrival at altitude and begin to decrease in severity around the third day.

The symptoms of Mild AMS include:

Headache
Nausea & Dizziness
Loss of appetite
Fatigue
Shortness of breath
Disturbed sleep
General feeling of malaise
Prevention of Acute Mountain Sickness.
If possible, don't fly or drive to high altitude. Start below 3,000 metres (10,000 feet) and walk up.
If you do fly or drive, do not overexert yourself or move higher for the first 24 hours.
If you go above 3,000 metres (10,000 feet), only increase your altitude by 300 metres (1,000 feet) per day, and for every 900 metres (3,000 feet) of elevation gained, take a rest day to acclimatise.
Climb high and sleep low! You can climb more than 300 metres (1,000 feet) in a day as long as you come back down and sleep at a lower altitude.
If you begin to show symptoms of moderate altitude sickness, don't go higher until symptoms decrease.
If symptoms increase, go down, down, down!
Keep in mind that different people will acclimatise at different rates. Make sure everyone in your party is properly acclimatised before going any higher.
Stay properly hydrated. Acclimatisation is often accompanied by fluid loss, so you need to drink lots of fluids to remain properly hydrated (at least four to six litres per day). Urine output should be copious and clear to pale yellow.
Take it easy and don't overexert yourself when you first get up to altitude. But, light activity during the day is better than sleeping because respiration decreases during sleep, exacerbating the symptoms.
Avoid tobacco, alcohol and other depressant drugs including, barbiturates, tranquillisers, sleeping pills and opiates such as dihydrocodeine. These further decrease the respiratory drive during sleep resulting in a worsening of symptoms.
Eat a high calorie diet while at altitude.
Remember: Acclimatization is inhibited by overexertion, dehydration, and alcohol
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