YOM KIPPUR: THOSE NOT OBLIGATED TO FAST

In Deuteronomy / Devarim 4:15 it is written:

Deuteronomy 4:15 And you shall be exceedingly careful to guard your lives ...

The purpose of keeping the commandments is in order to live and guard our lives. Therefore, it is forbidden to fast on Yom Kippur for those who would suffer an adverse medical condition as a result of fasting.

In addition, the rabbis teach that those under nine years of age should not fast on Yom Kippur.