A Jewish boy is traditionally obligated to fast on Yom Kippur at age 13.
This age corresponds to the religious milestone of bar mitzvah, when a Jewish male becomes responsible for observing the commandments. In traditional Jewish practice, boys generally begin observing the full Yom Kippur fast at 13, while girls traditionally assume this obligation at 12, associated with bat mitzvah.
Yom Kippur fasting is one part of a broader set of restrictions traditionally described as five “afflictions”: no eating or drinking, bathing, anointing with oils or lotions, wearing leather footwear, and marital relations. The complete fast runs from before sunset until after nightfall the following day.
Age rules should never override health. Jewish law treats preservation of life, pikuach nefesh, as paramount, so someone whose health could be endangered must eat or drink as medically necessary. Children below the full obligation may sometimes practice shorter fasts under family and rabbinic guidance. The common mix-up with age 12 reflects the traditional age for girls, not boys.