It is not the lack of fitness that prevents people from reaching the summit of Kilimanjaro, it's altitude sickness!
It is impossible to say for sure as altitude affects different people in different ways. Men, especially those who are young and fit, appear to be more susceptible than women because they will often ascend at a faster pace. Older people seem to be less susceptible because they tend to ascend more slowly.
Signs and symptoms of altitude sickness include:
Here are some tips to help your body acclimatize: