Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.114.

All scholars agree that fasting is obligatory upon every sane, adult, healthy Muslim male who is not travelling at that time. As for a woman, she must not be menstruating or having post-childbirth bleeding. People who are insane, minors, and those who are travelling, menstruating, or going through post-childbirth bleeding, and the elderly and breast-feeding or pregnant women do not need to observe the fast.

For some, the fast is not obligatory at all, for example, the insane. In the case of young people, their parents or guardians should order them to fast. Some are to break the fast and make up the missed days of fasting at a later date, while others are to break the fast and pay a “ransom” (in which case, they are not obliged to make up the days they missed). We shall discuss each group in more detail.