I mean don't Usagi, Mamoru, and the Senshi stop aging at some point? When you consider that it makes sense to me that they wouldn't care about Chibi Usa not aging because that's basically normal for them. Why should they care?
Because 900-year old Chibs is introduced as a concept bundled with commentary that it’s not normal.
And when Minako says “Well if she doesn’t ever get powers, how can she take over the throne” (which is utterly fucked thinking but whatever), Endymion says:
I’m following on from what comes with the exposition. I mean if they’d come out and said “She stopped aging 900 years ago, but that’s normal now”, then I’d still think it’s stupid, but fair enough, that’s just how things are. But it’s NOT normal, More, Endymion’s response indicates it’s a real cause for concern (if for absolutely none of the reasons it should be, such as WHAT IS GOING ON WITH YOUR CHILD THAT HAS CAUSED THIS DEVELOPMENTAL ROAD BLOCK).
THIS IS TAKEUCHI’S PLOT POINT I DIDN’T CREATE THIS SITUATION