A spot of blood is not a sign of reaching puberty
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?
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.