lmao, it's obviously a rule work around. Just look how they act and are treated. The non adult looking characters are clearly modeled to be middle schoolers at best. Anne has a elementary mindset. Of course there's a few girls like the tanned swim club girl, Mae and the delinquent girl who look older than the rest of the students but usually in a countryside town, schools have all 3 lvls of education in one place.
In short, most games like these SAY the characters are 18+ but it's obvious that is bs. They are just doing what they can to stay legal while making shota based and loli based games.
As to your random thought, depends on the artstyle. More anime styled or western cartoon styled characters is just going to look young. Usually short yet adult looking characters come from a more realistic style like Aurelia. However for games like this, short looking adults isn't what they are aiming for. It's catering to the onee x shota (older girl with younger guy) crowd, thus the mc is usually elementary or middle school looking while the girls are high schoolers to middle-aged women.