Purity: Uncertainty of Age and Menstruation
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 436:
If a girl who does not know whether she has completed nine years of age experiences bleeding that does not have the attributes of ḥayḍ, it is not ḥayḍ. If it has the attributes of ḥayḍ, then considering it to be ḥayḍ is problematic [i.e. based on obligatory precaution, it must not be considered to be ḥayḍ], unless one is confident that it is ḥayḍ, in which case the girl will be considered to have reached the age of nine.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)