A main criterion for female leads, even in animated movies, is that they should be young—or at least, youngish. It is one of the reasons why I like Howl's Moving Castle so much, because for the most part, we spend time with Sophie, the lead, when she is an old woman, subverting the typical tropes that the movie itself can be said to deal with: that of beauty and vanity.
Sophie Hatter, who considers herself the plainest of her siblings, finds herself cursed by the Witch of the Waste. Her curse is this: her insecurities about her looks are externalized onto her skin, so she appears and feels like an 80-year-old woman. It is how she meets Howl, the love interest and titular character, who is a pretty but vain man who avoids his duties as a sorcerer during wartime in favor of making potions to make himself look beautiful.
Sophie's role within the story isn't what one would assume it to be. Often, when women of a certain age are portrayed in media, they are shown in maternal, community-based roles that remove them from any possibility of adventure, or even love. However, Howl's Moving Castle subverts this very concept. Yes, Sophie is portrayed as a maternal, caring figure, hired as a housemaid for Howl's castle, and tending to both Howl and his young apprentice Markle. In one scene, she pretends to be Howl's mother in order to excuse him from having to be conscripted into war—seemingly typical roles an older woman would be expected to uphold.
However, in adding further context and detail, the movie subverts this. Because one of the main themes of the movie is about vanity, especially vanity that comes from upholding a certain standard of beauty, Sophie's "oldness" isn't depicted to hold her back. Rather, it liberates her. She is shown to be transformed from a young, timid, mouse-like woman, into someone whose older appearance now liberates her from certain standards society holds for women: she not only travels alone, but is actively made a part of the more adventurous aspects of the plot. Her "oldness" isn't used as a plot device; it is the plot, allowing her to not only empathize with the Witch of the Waste, whose greatest fear is aging (the very same curse she puts on Sophie), but also allows Sophie to come to terms with her own fear of not being beautiful enough.
In the end, the concept of growing old, especially for women, is seen as such an end-of-the-world issue, when in reality, it is simply another role that women are meant to conform to, even when, paradoxically, they will never be able to permanently adhere to. Howl's Moving Castle, in both depicting its protagonist as an older woman, and pairing that choice with the theme of beauty, vanity, and (magical) body alterations, can add to the commentary that has plagued visual media since its early days. Everyone ages. But that doesn't make them any less beautiful, or any less human.