Altitude is defined on the following scale:
- High altitude: 8,000 - 12,000 feet (2,438 - 3,658 meters);
- Very high altitude: 12,000 - 18,000 feet (3,658 - 5,487 meters); and
- Extremely high altitude: 18,000+ feet (5,500+ meters).
Most people can go up to 8,000 feet (2,438 meters) with minimal effects. If you have been at that altitude before and had no problems, you can probably return to that altitude without problems if (and only if) you are properly acclimatized.
If you have not been to high altitude before, caution is strongly recommended. No specific factors such as age, sex, or physical condition are known to correlate with the susceptibility to altitude sickness although there is a decreased incidence among those 50 to 60 years or older and women may be at a slightly higher risk than men. Some people are simply more susceptible than others. There is no telling, so caution is the better part of wisdom.