Halakhah for Yoma 146:6

And the law of the sick person who is in danger, that we feed him according to an expert physician, or according to himself - and even if the physician says that he does not need [to eat] (Yoma 83a); the law of a pregnant woman that smelled [food] (Yoma 82a, 83a); the law of one who was seized by bulimia; and the law of minors - whether a boy or a girl - from what time do we we afflict them all of the day rabbinically, and likewise from when do we teach them [to fast] with [some] hours. And the principle of the the matter according to a few of the commentators is that two years before the adulthood of a healthy child, they complete [the fast] rabbinically; and two years before that, we teach them [to fast] with [some] hours. And some explained that it is one year before adulthood that they complete [the fast] rabbinically - whether a boy or a girl. And some explained that only a girl alone completes [the fast] rabbinically one year before adulthood, but a boy does not complete [the fast] at all rabbinically. And adulthood for a boy is thirteen years and a day, and for a girl is twelve years and a day. And what is the law about washing for the king and the bride on this day (Yoma 73b).