You can create older characters in Koikatsu, by default yes they are younger looking as its a school setting.
Still it's easy to create older characters in Koikatsu, its not the engine's fault for the loli's in the OP to be sure.
No judgement though here, just saying as someone who is using it for my story as I'm creating alot of both.
It's the very skewed used of the word "loli" nowadays, where I assume your stuff too might be too "loli" for some people as well even because of the cutesy moe stylings of it with koikatsu. Not quite what "loli" actually means, but what I see it often used for, especially by people less informed of the terms. Even see chibi versions of characters referred to as loli even though that's a different concept. Of course you can mess with proportions for more mature looks, Tsukachan's Janis here is still very buff, even in koikatsu, but essentially, I had a hunch "loli" here, especially with the hesitation it was used, was more in reference to the cutesy style of the game rather than the actual loli-isms.
...if that makes any sense.