Just had a little what if story idea for Hotaru if Non Non Biyori wasn't a simple SoL comedy.
Rather than a ten year old girl who just happened to grow very quickly what if Hotaru actually was a girl in her late teens who developed a disorder that made her mentally regress to that of a young girl. Her family moving out to the countryside to make it easier for her to live her life away from the stresses of the city.
It could just be some kind of medical condition or if one wanted to go really dark with the idea the mental regression could be a form of PTSD after being sexually assaulted.
Edit: Keeping with the rape line of thought Hotaru could start having flashes of memory or dreams she doesn't understand but feel oddly familiar. Finding a stash of letters and or photos her parents has carefully hid that mention a family member she doesn't remember. Ultimately it is revealed said family member, an uncle maybe, whom she really trusted raped her which caused her mind to break and so she mentally regressed to that of a ten year old. The family member who did the deed either in prison or dead.
Not the first time my mind came up with dark what if stories like this. I remember watching the Denpa Onna series hoping to one day see the truth about what happened to Erio. What caused her disappearance and her suddenly believing herself to be an alien. And so my dark imagination came up with the what if story that she was abducted by a pervert being a very cute girl who often went out alone at night. Her being kept as a sex slave for several months until she escaped but her mind was left broken by the ordeal.