Age restrictions are pretty irrelevant when it comes to kigurumi. They are after all, not only cute, but also very comfy to wear. Why would we even think of sleeping while wearing one if it doesn't make you feel cozy?
"Why are you acting like nothing's different here?!"
That being said, it is indeed quite a lot easier to imagine a child, than an adult, wearing one. Mashiro Mitsumine did have this very same thought, although she quite ended up blowing her cover regardless, all the while wearing one anyway.
But first, let's provide some context. While it was instantly presumed that she is younger than his brother Hakuya, she attempted to make it seem that the age gap wasn't that wide.
"What? I'm a first-year high school student no matter how you look at it."
But as Kobeni pointed out countless times, Mashiro's demeanor never actually played the part, dwarfism or not. It is even quite worsened by the fact that the very blue cat kigurumi Benio ordered specifically for her is for kids.
Finally, however, because Hakuya's tongue slipped after much conscient pressure, she at long last revealed her real age, which was ten. Specifically soon-to-become ten on that year.
"...I'm actually ten. No, actually, I'm about to become ten this year."
Kobeni, quite understandably, had a hard time believing this. No, not because she's nowhere near the age of being a high school student. But she believed that the number was even lower.
"I'm telling the truth! I admit that I may look even younger than a ten-year-old but..."
This is much to the depression of our beloved petite Inugami girl, who knew very well that this is a hereditary case, something that is absolutely out of her active control.
"You don't seem to be making progress there."
In that case, you really have to wonder what exactly it is that her subtly-entranced schoolmates see when she's in school. Especially with such an oversized uniform.
"Thank you for taking care of Hakuya and Mashiro."
Yes, if the wonderfully pastel-like colorful palette doesn't give it away for you, this anime is yet another adaptation made by kigurumi anime expert Doga Kobo. Engaged to the Unidentified (未確認で進行形) is as typical as any slice-of-life comedy anime as it can get. That's right, comedy, not romance comedy. Despite that, the show is somehow still quite entertaining to watch, perhaps due in part to just how cute Hakuya's interactions get over time with Kobeni and the modern world around him.
Come to think of it, maybe part of the studio's choice criteria in its anime adaptations is finding kigurumi in the source material? We may never know.