Praise be to Allaah.
Whatever
the case, you have to fast once you have reached the age of puberty,
which in the case of a girl is when she starts her period, gets pubic
hair, experiences erotic dreams or gets pregnant. Having one’s periods
is one of the signs of puberty, so you have reached puberty and are
accountable, so you have to fast in Ramadaan. In any case, you have
to make up the month that you did not fast; you cannot clear yourself
except by making up those fasts and repenting, because you were accountable
at the time when you did not fast. Your family made a mistake by being
too lenient with you. You are not a little child, and you have to repent
from this negligence. And Allaah knows best.
From Fataawa Samaahat al-Shaykh ‘Abd-Allaah ibn Humayd, p. 176