Maisie is 18.
She is small and could pass for 12 (or I have what she used to look like in season one of Game of Thrones stuck in my head.).
How old is Ashildr?
1. Had she started puberty before the medkit was installed, because it's unlikely that she wouldn't have after it locked her biology.
2. If she were of child bearing age (did you see a husband?), with it's own native species (thinking about Farescape) the question of pregnancy probably has several answers to the technology inside Ashildr, but in the weirdsville that is her body, the medkit would likely treat any pregnancy like a biological infestation or parasite and deal to it immediately.
3. Just because she can't make her own babies, if she can't make her own babies, adoption is still a thing, if she feels an urge to be a mother. Or she could look after follow her nieces and nephews family trees from the village as extended family?