Everybody responds differently to altitude. It is not directly related to fitness that is why it is important that you get tested.

There is good evidence that things you cannot change, such as age and gender, affect your chances, although doctors cannot explain why (we have our theories as to why). Females are less likely to experience AMS than males. At moderate altitude, young people are more likely to suffer AMS than their elders: the risk decreases with age in an almost straight line.