Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 616…
“ A girl one day past twelve years and a boy one day past thirteen years who have [pubic] hair are treated like adults for every commandment and they have a Torah-obligation to fast the full day. However, if they do not have hair, they are still children and they only fast by rabbinic law. Rem"a: Even if they are soft and weak, they must fast because we worry that the hair fell out (Terumat Hadeshen 155) . We do not have children under nine years of age fast on Yom Kippur so as not to endanger them. ”
