Age of puberty in Hidaya:
The puberty of a boy is established by circumstances, or upon his attaining eighteen years of age; and that of a girl by circumstances, or upon her attaining seventeen years of age.
It is to be observed that the earliest period of puberty, with respect to a boy, is twelve years, and with respect to a girl, nine years.
So if one hits puberty for example at 11, is it obligatory to fast in Hanafi fiqh or is it obligatory to fast when you are 12 in Hanafi fiqh? My family which is Hanafi says that it is obligatory to fast when you are 12 so I'm thinking this might be why.