The real question is how they got Tatsuta to go along with this.
Eh, really? Seem to me just going "hey wanna do something trollish that'll embarrass the shit out of the Admiral?" would be all it takes to get her on board.
I mean they even mention the whole "they don't change physically" on the next page. That was always my assumption as well, which also meant I always tended to judge 'adultness' by bearing and behavior vs appearance. Which is why to my mind RJ is definitely an adult contrary to above. She's pretty definitively mature and put together in a way many of the more childish girls just aren't. Consider for instance how she's usually portrayed to interact with someone like Saratoga or even Houshou as a peer. It's not about class to me either, there are some destroyers I would consider adult as well with Shigure coming to mind as the most obvious example (ironically Yuugumo herself would be on the borderline to me, but leaning toward adult).
Hey...Experimental ResultsSo it appears it has little to do with one's outwards appearance, then...?I do believe that the borderline is right around the light cruiser area.Why do I gotta be the butt of the joke, hey...?
Explain.Tatsuta → AdultTaihou → AdultKongou → AdultRyuujou → ChildMurasame → ChildTenryuu → Child