See, end the end you sort of just have to chalk it up to that even though it makes no fucking sense. Like, it doesn’t even add up if you count her as being alive during the time she was traveling/passing through.
The canon “explaination” is she stopped aging because of her close exposure to the Silver Imperium Crystal, kind of like how the citizens of Crystal Tokyo get super long life spans…but that doesn’t make any sense either because the crystal was fucking inside Usagi for the first 14 years of her life and she still aged. I guess you could argue it wasn’t as powerful then, but it wasn’t that powerful in the timeline Chibiusa first came from either because everything went to hell when the Black Moon Clan attacked and the Future!Senshi we saw weren’t even Super forms, much less eternal.
There really is no reason for it. My pet theory is that Naoko wanted Usagi and Mamoru to have a few years of Happy-Normal-Family-Life before being crowned Queen and King of the galaxy, but you know, R had to happen so Chibs had to be a kid in Crystal Tokyo.
Whatever the reason (and that’s using the word liberally), I just tend to ignore it. Like Mamoru creepily kissing drunk passed out Usagi in Classic. It just doesn’t fit with the characters or the plot at all.