I have a sneaking suspicion that nayuta has taken the drug, and due to her referring to both mahiro and mihari as onii and onee respectively (which is used when talking too older brothers and sisters or brother and sister-like people respectively), i think they're going to be younger than both of them. Of course, that doesn't say a lot... Also it might just be a combination of them adapting to their new body and knowing the original ages and genders of both of them.
Edit: wait, i just read the bottom text at the credits page, and that would make sense, but i still dont think that is the case, eh, we'll find out next month maybe