Do you remember the 1990s, when Kate Moss made the term "waif" fashionable? Or for that matter, do you remember 1950s media, which featured "gamines" -- sexualized women who looked quite young?

https://en.wikipedia.org/wiki/Gamine

The issue looks weird to me because I live in East Asia, where I see a lot of cute, petite ladies (well over 21 years of age) every day. Many of those ladies are ultra-feminine and some of them talk in exaggerated voices. The fact is that women want to be pretty. So you get a complicated social landscape as girls and women try to be pretty in various ways that seem reasonable to them. However, anime is not likely to help us understand the real-world situation. Try watching live-action TV from Japan, or Hong Kong, or India and note how the adult actresses from those countries are much, much more feminine that actresses from Hollywood.

https://asianwiki.com/Serena_Motola

This lady was born in 1998, so she was in her 20s when I first saw her on TV, but she still looks like a pale, ethereal waif.