There were prenatal growth problems, where one month she was below the growth curve but the next month she had caught up her growth. Of course I can only speculate, but there could have been an incident at 6 months of age that triggered the stop in development (think for instance a virus infection that put a strain on her system).
There are cases known where mono-zygotic or di-zygotic twins both carry the same mutation(s), but disease only manifests in one of the twin. Twin pairs showing discordance of phenotype in adult Gaucher's disease
Another speculation is that in other organisms there are different developmental stages that each require specific triggers to start that program, a defect in such a trigger would cause a developmental stage to be repeated or skipped. Maybe there is such a switch in humans that causes development past her age, but I don't have any evidence for that.