Praise be to Allah.
A woman is deemed to have reached puberty when one of four things happens:
1 – When she reaches the age of fifteen.
2 – When her pubic hair grows, which is hair around the private parts.
3 – When she emits maniy (fluid released at climax).
4 – When she starts to menstruate.
If one of these four things happens, then she has reached the age of puberty and she is now regarded as accountable, and she is obliged to do acts of worship just like adults. If a woman did not know that puberty begins in this manner, then there is no sin on her for not fasting, because she was unaware, and there is no sin on the person who is unaware so long as he was not negligent about learning or finding out when he was able to do so. But she must hasten to make up the fasts that she missed because when a woman reaches the age of puberty, she becomes accountable and she must make up the fasts that she missed during the time when she was accountable. She must strive to work out how many days she missed after she reached puberty, and hasten to make them up so there will no longer be any sin on her.