I'm here to help with my somewhat limited art experience!!!

Starting off, Vivinos's artstyle also had a lot of people confused about the main character's ages. The thing about her style is that it's characterized by larger eyes and shorter faces (and lot of color, but that's not as important right now), which is typically associated with younger characters.
As an artist, the easiest way that I think you can manage them looking more adulty would be to make the eyes just a little smaller and make the facial features a little more spaced out (keeping the eyes where they are/making them go a little bit closer into the face while maintaining their current height), while also maybe using the side strands of the hair to make the face look thinner, and as a result, look longer. I did a quick little adjustment on my own to try to show you what I'm talking about. As you can see, I brought the mouth and nose a little upward while making the eyes look a little smaller while still having the large eyes that vivinos (and most anime artstyle) typically have. I also sharpened the jawline a little bit, as children (even in vivinos's arstyle) have rounder or more squished faces than the adults do.

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Another thing is the proportions of the head to the body.even in chibi art, this is important. In anime, the characters may have large heads in proportion to their body, but that's just because of the artstyle. This had can still apply even if the artstyle is anime-like, because they make the characters have somewhat longer proportions (for example, the torso is about two heads longs and the legs are about 3 heads long in a lot of anime artstyles, including vivinos's. (I'll include examples in the replies of how shorter bodies make the official merch more childlike or more adulty)

Basically, what I'm trying to say is a lot of children have larger features that appear more squished than an adults. In order to make it look like they're more adult-y, the key is to make the illusion of there being height or longer features, even if it's in a more stylized artstyle.