How Up Do You Need To Be To Get Altitude Sickness?

Altitudes are classified in various ways such as high altitude being somewhere between 8,000 to 12,000 feet, very high altitude is about 12,000 to 18,000, and extremely high altitude is anything above 18,000 feet. You need to get up to at least around 10,000 feet to get altitude sickness. One of the symptoms which can warn you about altitude sickness is feeling nauseated and weak.

As such there are no physical barriers for altitude sickness i.e. age, sex and physical condition do not matter. At high altitudes, the level of oxygen in the air decreases by about 30-40 percent. To adjust with high altitude your body should first get used to the atmosphere. Every time a person climbs higher on the mountain his body needs to re-adjust itself to the new altitude so that no problems occur. If the body can't cope with the pressures of high altitude the result can also be death. It is therefore recommended during mountaineering expeditions that one doesn't climb too high too fast.

At 9,000 feet, respiration speeds and deepens as the body senses less oxygen. The brain swells slightly, causing headache and nausea, The kidneys excrete a hormone that triggers the production of more red blood cells. Seventy-five percent of the oxygen at sea level is available.
At 18,000 feet, the lungs expel more carbon dioxide, disrupting the blood's pH balance. The kidneys discharge more water to correct the blood's acidity, causing dehydration. Fifty percent of the oxygen at sea level is available.
At 29,000 feet -- the summit of Mount Everest -- the heart pounds, even at rest. Auditory and visual hallucinations, confusion, and slurred speech begin. Pulmonary and cerebral edema are likely. Thirty percent of the oxygen at sea level is available.
This altitude is called the "death zone" for good reason. The body begins to feed on itself, resulting in brain damage. So one must get up and down as quickly as possible.
The highest permanent human settlement is La Rinconada, Peru, at 16,730 feet. Andean people have more red blood cells, greater lung capacity,  and smaller bodies, which have fewer capillaries to transport blood.