The first condition for one to be obligated to fast is to be Muslim. In this world we do not request from the non-Muslim to fast; however, in the Hereafter, he will be punished for
neglecting to fast–as he will be punished for his blasphemy. Rather, the obligation on the non-Muslim is to embrace Islam, then among many obligations, is to fast Ramadan.

Secondly, the Muslim must be pubescent. The child is not obligated to fast. However it is an obligation on the guardian of the child to order him or her to fast once he or she is 7 (lunar)
years old, with the condition that the child’s body can withstand fasting, without being harmed.

Also, for one to be obligated to fast, he must be of sound mind. Fasting is not obligatory on the insane person.

Fasting is not obligatory on a person whose body cannot tolerate fasting, either due to old age or severe illness. Moreover, if a person may be harmed by fasting, that is, as a result of
his fasting his sickness may worsen or he may die, then fasting is prohibited for him. The person who does not fast because of old age or an illness he is not hopeful to be cured of, does not have to make up the missed days of fasting. Instead, he pays an expiation to a poor Muslim for every day of fasting he missed. He can pay the expiation for every missed day to the same person or he can choose to pay it to different people. The expiation is a pair of average-sized hands cupped together (mudd) filled with the most common staple food of the town. In the United States, for example, the most common staple food is wheat.

Fasting is not obligatory on the menstruating woman or the woman who is in her postpartum bleeding period. In fact, it is unlawful for them to fast. The woman who missed days of fasting during Ramadan for these reasons has to make up each missed day.

Also, the pregnant woman is allowed not to fast if she fears harm may come to her or her baby from her fasting. This includes the breast-feeding woman as well. She is allowed not to fast if she fears harm may come to her or her baby from her fasting. Both the pregnant and nursing women must make up every missed day of fasting. If the reason such a woman did not fast was solely the fear of harming her baby, then in addition to making up the missed days of fasting, she must pay an expiation This expiation is a pair of average-sized hands cupped together filled with the most common staple food of the area where she lived, for each day she missed.

The person who is traveling a walking distance of two or more days (about 80 miles) is allowed not to fast–provided his traveling is not sinful. This traveller is permitted not to fast even though he would not encounter hardship during his trip–such as if he crosses this distance quickly, by plane, or comfortably, in a car or a train. The matter of traveling is not based on the hardship; rather, it has to do with the distance. This facility for the traveller was mentioned in the Qur’an.

Allah said in Surat al-Baqarah, Verse 185:

ومن كان مريضا أو على سفر فعدة من ايام أخر

If one is sick or traveling, then one is allowed not to fast and one makes up the missed days later.