Blood pressure does run lower in younger people; the range of blood pressure for a 13 year old girl should run from 103-110/62-65. The BP you listed is significantly lower.
This finding could be caused by a couple of things. If the girl has no other symptoms, she could be a perfectly normal variant and have low blood pressure for her age. Two, if she has been bleeding (i.e. has she just started to menstrate?) heavily, that could explain a transiently lower blood pressure. Lastly, illness and dehydration (i.e. has she had a bad cold or stomach virus) can cause low blood pressure.
An appointment with the child's pediatrician would be useful to help determine why her blood pressure is low.
To your health,