In Islam, the age at which girls and boys become duty bound is contingent upon the signs of bulugh (coming of age), which is seminal discharge in boys and menstruation in girls. However, in Islam, in addition to these natural biological signs, an age limit has also been determined. If this age is reached and those signs haven't been seen yet, they will by duty bound and are responsible for their religious duties and actions. Therefore, it isn't true that the Sunni school says that bulugh begins with puberty, while the Shia believe it to be at 9 years of age. Rather, if a girl experiences menstruation, or becomes pregnant, she will be baligh according to all schools of thought, even if she hasn’t reached the age that has been determined as the limit by the Sunni school of thought.
Bulugh is a natural state that both males and females experience some time in their lives. In this stage, the body undergoes major change and growth spurts. These physical changes vary depending on the individual, the climate of where one lives in, one’s nutrition, etc. Changes such as in voice, the growth of coarse hair around the private parts, seminal discharge in males, and menstruation in females. In Islam, the time when one becomes duty bound has been set on when one experiences the signs of bulugh (most notably, seminal discharge in males and menstruation in females). However, in Islam, in addition to these natural biological signs, an age limit has also been determined. If this age is reached and those signs haven't been seen yet, they will by duty bound and are responsible for their religious duties and actions.
Both the Shia and Sunni schools of thought agree that if a female becomes ha’idh (experiences menstruation) she will be baligh (has come of age religiously and is duty bound). There are other signs also, but since they are not agreed on, we will not mention them here. Therefore, it isn't true that only the Sunni schools says that females become baligh from physical puberty and that the Shia only believe that to take place upon 9 years of age (although in some places, females’ puberty coincides with them reaching 9 years of age), rather, if a girl experiences menstruation or becomes pregnant, she will be duty bound according to all schools of thought, even if she hasn’t reached the age limit determined by the Sunni school of thought. So, when it comes to the signs of bulugh, there is no difference between the Sunni and Shia schools of thought. The only difference between the two schools is regarding the age limit.
Bulugh by means of age
There is a difference regarding the age at which girls become baligh between the Shia and Sunnis, there is difference within the Sunni school also regarding this issue.
The famous verdict on this issue among the Shia scholars, is that the age for girls is 9 (lunar years) and for boys is 15 (lunar years).
The Sunni view on the age of bulugh for girls:
Hanafiyah school: The age for both girls and boys is 15 lunar year. This is despite the fact that according to Abu Hanifah himself, the age for boys was 18 lunar years, and for girls, 17 lunar years.
Malikiyah school: The end of 18 lunar years.
Shafi’iyah school: The end of 15 lunar years.
Hanbali school: The end of 15 lunar years.
Hanafiyah school: The end of 15 lunar years.
 See: Jazayiri, Abd al-Rahman, al-Fiqh ala al-Madhaahib al-Arba’ah wa Madhab Ahl al-Bayt, vol. 2, pg. 412, Dar al-Thaqalain, Beirut, 1419 AH.
 Of course, they have assumed the least age a girl can experience menstruation to be nine years. See: Kitab al-Taharah (of Imam Khomeini, new print), vol. 1, pg. 9.
 Al-Fiqh ala al-Madhaahib al-Arba’ah wa Madhab Ahl al-Bayt, vol. 2, pg. 412.
 See: al-Fiqh ala al-Madhaahib al-Arba’ah wa Madhab Ahl al-Bayt, vol. 2, pg. 412.
 See: Tusi, Muhammad bin al-Hasan, Al-Mabsut, vol. 3, pg. 37; Sharaye’ al-Islam, vol. 1, pg. 179; Jame’ al-Maqasid, vol. 5, pg. 180; Tadhkirah al-Fuqahaa’, vol. 14, pg. 188; Idhah al-Fawa’id, vol. 2, pg. 50; Kashf al-Rumuz fi Sharh Mukhtasar al-Naafi’, vol. 1, pg. 552.
 See: al-Fiqh ala al-Madhaahib al-Arba’ah wa Madhab Ahl al-Bayt, vol. 2, pg. 412; Fatawaa al-Azhar, vol. 10, pg. 426; Al-Mowsu’ah al-Fiqhiyah, vol. 2, pg. 332, Al-wuquf wa al-Shu’un al-Islamiyah bi al-Kuweit,: “وقد اختلف فی سنّ البلوغ : فیرى الشّافعیّة والحنابلة وأبو یوسف ومحمّد من الحنفیّة ، وبرأیهما یفتى فی المذهب ، والأوزاعیّ ، أنّ البلوغ بالسّنّ یکون بتمام خمس عشرة سنةً قمریّةً للذّکر والأنثى ' تحدیدیّة کما صرّح الشّافعیّة ” Ibn Umar.