Admittedly I don't watch enough anime to have full context, but in numerous situations I've seen what's depicted as a little girl is actually a woman 100+ years old. Is there a reason for this characterization? Is it a plot device, or is there cultural/historical reasons? There's a wiki article that talks about examples but not the history or reason for the trope: https://tvtropes.org/pmwiki/pmwiki.php/ReallySevenHundredYearsOld/AnimeAndManga
Here's a relevant hilarious video conveying how age is often inconsistent with maturity or appearance: https://www.youtube.com/watch?v=XGxCMyLm6js