Why are old anime women often depicted as young girls? 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
I suppose this depends on your definition of "often". It also heavily depends on the kind of show you're watching.

If you're watching a slice-of-life anime which has its characters closely rooted in the modern reality as opposed to anything supernatural, then older women are typically drawn closer to what our expectations would be.

If you're watching an anime which is rooted in more supernatural, fantasy or "other-world" (isekai), then you're going to run into this a lot more often.

For instance, it's reasonable to say that an older human woman would look closer to what we would expect, since we have a more familiar reference point.  In a fantasy setting, it's not unrealistic to suggest that an older [insert race/species here] woman would look to us as if they were a much younger human woman.