Fasting is not obligatory on a few people :

Fast is not obligatory on a person whose body cannot tolerate fasting, either due to old age or because of severe illness. Fasting is also not obligatory on the mensurating women (infact it is unlawful for them to fast). A pregnant women is allowed to escape fasts if she fears that it would harm her or her baby. The one who is travelling a distance of two or more walking days is allowed not to fast. The child is not obligated to fast (until he/she is 7 years). Fasting is also not obligatory on the insane person.