A spot of blood is not a sign of reaching puberty
Question

                My mother’s friend has a daughter who is 13 years old. She found a drop of blood in her underwear two years ago. The mother advised her to fast that previous Ramadaan; as she thought that the drop of blood is a sign of puberty. The mother did not notice any sign of puberty on her daughter since that time. The mother is asking now if her daughter must fast, bear in mind that her daughter has already fasted the previous Ramadaan.
Praise be to Allah.It seems – and Allaah knows 
    best – that a spot of blood in the case of this girl cannot be regarded as 
    menses, rather it is temporary bleeding that may be the result of sickness 
    and the like. That is for several reasons: 

    1-This bleeding did not happen 
    again after that. For two years until now she has not seen any real menses. 
    This clearly indicates that she has not started to menstruate yet.

    2-One or two spots of blood is 
    not regarded as menses.
Shaykh Ibn ‘Uthaymeen (may 
    Allaah have mercy on him) was asked – as it says in Fataawa al-Mar’ah 
    al-Muslimah (1/137) – about a woman who experienced spotting in 
    Ramadaan, and this spotting lasted throughout the month, but she fasted. Is 
    her fast valid? 
He replied: 
Yes, her fast is valid. As 
    for this spotting, it does not mean anything, because it comes from a vein. 
    End quote. 
This girl is not 
    menstruating and she has not reached puberty, so she is not obliged to fast, 
    but she should get used to fasting so long as she is able for it, so that it 
    will be easier for her in the future. If she fasts, her fast is valid and 
    she will be rewarded for it in sha Allaah. 
And Allaah knows best.