In which month is it impossible to become Bas Mitzva?
It is impossible to become Bas Mitzva in Adar Rishon. If a girl is born in a year where there is only 1 Adar then her Bas Mitzva in a year where there are 2 Adars would be in Adar Sheini. If a girl is born in Adar Rishon - 12 years later would always have only 1 Adar, thus you can never have a Bas Mitzva in Adar Rishon.
Answer Here with extensive treatment of the topic in other Posts: