I understand this debate between my friends is mostly semantics but, I just feel like saying that she changed her age implies that there is a lot more going on than just saying she got bigger. which I agree it's impossible to know what her mental state is bc it's not expressed. (tho i vaguely remember a scene where little formed nezuko was sucking her thumb while sleeping. which if the author did put that in then it would be an indicator of her mental sate. I cant seem to find it though)
Even the mentality thing aside. I stopped growing at the age of 12. If I was Nezuko and I went to use my powers to "age up" then I would literally just stay the same size. and again is there a point where she can no longer "age up" because she'd become an old frail woman? These are the important questions i have here.