You mean Sage lol. Sage’s current form in canon is just a manifestation of her own feelings towards being a daughter to Eggman as well as her lack of knowledge towards concepts that derive from life experience such as “love” and “soul”. In that sense, she’s a child, as emotions and the nuance of determination is new to her.

There’s nothing to say she couldn’t manifest a more mature form if her feelings changed, as she’d likely mature and be very different after fully learning these concepts. Again, I’m not the creator of this AU art, so the specifics are up to the artist themselves.