Threw it out and had her returning from school with her brother and cousins. Which does not intrinsically make her older, but I managed to make her sound older. I think. The crucial thing is how she is a misfit and her relatives are unkind. (Hmm. . . I think she needs to be surprised at their all attending school that day. Did I start the story too late?)
Still, this is a story that works with a fairly limited age range. Young enough that she is not legally able to shake the dust of her family's home off her sandals without a great deal of effort, old enough that it's practical if the legalities are sorted out. (Pushing back about how her family beat her down is part of the character arc.)