The obligation of the Ramaḍān fast is contingent upon a person being both legally responsible (ʿāqil) and having reached puberty (bāligh). In the case of females, puberty is established with the onset of menstruation. Accordingly, once a girl begins menstruating, the Ramaḍān fast becomes obligatory upon her.
Conditions such as being too weak or ill to fast do not remove the obligation of fasting itself; rather, they relate to the permissibility of postponing the fast. If a person who is legally obligated to fast would suffer harm to their health by fasting, or if an existing illness would worsen, it is permissible for that person to delay fasting until a time when they are able to do so in good health.