Kakegurui features a cast of women that come in all sorts of shapes, sizes, orientations and personalities. Some are wheelchair-bound. Some are pop idols. Some are gambling addicts, as the show's title suggests. However, for all of the different kinds of girls who show up in the series, there's one oddity in the cast worthy of note -- the childlike Yomozuki Runa.
Where exceptionally pretty teenage girls seem to be the norm of Kakegurui, Runa for some reason has the appearance of a little girl. It's not uncommon for girls to appear underage in anime, but it always leads to questions like why they look so young and how old they really are. In the case of Runa, her age is never provided, so it's left up to the fans to infer it. Here's an estimate of her age based on the evidence made available.
The first conclusion that comes to mind when seeing a girl like Runa is that she belongs in elementary school or early middle school. It's not just her petite build that says this either. Her mannerisms, speech patterns and Kigurumi outfit are all indicative of someone in this age group. She's also generally seen playing video games and eating candy, both of which are often associated with younger kids. Assuming she's that much younger than the other students at Hyakkaou Private Academy wouldn't be unreasonable.
If Runa truly is a little kid, however, there would have to be an explanation for why she's already in high school, and in her senior year, no less. One possibility is that she's a genius who was allowed to skip several grades ahead. As a member of the Student Council and the leader of the Election Committee, she certainly showcases a greater level of intelligence than even regular high-schoolers.
Another possibility is that she's related to somebody in charge of the school. If this is the case, she wouldn't necessarily have to be a student to be allowed to freely roam the Hyakkaou campus. There are no scenes of her attending class, so there's a chance that this is the case. Unfortunately, there's no evidence of this in the narrative, so the theory is dubious at best.
The more likely answer, and perhaps the simplest one, is that Runa is a high schooler with a petite build. She isn't actually an elementary school girl -- she just happens to look like one because she's 4'3". There are plenty of explanations for this, including malnutrition and dwarfism. While the show never explains her height or her childlike appearance, this seems like the safest assumption.
Runa acting childish could just be adherence to the cutesy, or kawaii, aesthetic. In Japan, there's a particular emphasis on the appeal of kawaii that permeates the culture. Many Japanese women of all ages aspire to be kawaii, although it's of course the younger girls who have the most success in presenting it. In this regard, even if Runa isn't an actual child, her childlike appearance is the best fit for evoking feelings of kawaii, so she may just be rolling with it and playing the part.
Moreover, even if Runa were a child, that wouldn't explain the rest of the Election Committee. Like Runa, the members of this Committee have petite builds and sport Kigurumi. Either they're all extremely smart elementary school girls or they're all similarly short, cutesy high schoolers.
Runa Yomozuki is probably around 17 or 18; the general age range of third-year Japanese high-schoolers. Barring the possibilities of superintelligence or family relationships, she should fall within this same age group. Her kawaii approach to life could be construed as childlike behavior, but that doesn't necessarily have to be the case. She may not be a buxom beauty like most of the Kakegurui cast, but she could be much more like them than she lets on.