Bismihi ta'ala 

According to the mainstream view of Islamic jurists, a female reaches the age of religious maturity, or bulugh and becomes religious duty-bound (mukallafah) at the completion of nine lunar years.

This is irrespective of whether she has reached physical puberty, or not. 

The evidence for this are authentic traditions from Ahlul Bayt (a.s.). As Muslims we believe that our religion has taken into consideration all aspects of a human being, whether it be physical, social, mental or spiritual. It is for this reason that we must adhere to the legislative rulings conveyed to us through our jurists, as they have deduced these rulings through the primary sources of Islam. 

I would like to further add that this is indeed an honourable privilege for females, as it means Allah ta'ala has blessed females with something quicker than males. There are two conditions for takleef becoming obligatory on a believer: 'Aql, which means rational comprehension, and qudrah, or ability.

A female will surpass a male in acts of worship and obligations and devotion to the Almighty, which is also another valuable privilege. She becomes responsible, accountable and independent in the religious side of her life. She will develop quicker than a male, and mature in all ways faster than a male. 

Furthermore, we must not mix between puberty, and entering into the age of takleef.

And Allah knows best.