Why do Conan and the Detective Boys call Ai "Haibara-san"? Since a few months, I'm watching Detective Conan in Japanese with subtitles (because only the first 308 episodes are available in German). After a few episodes, I did some research on japanese names, since "-san", "-chan", "-kun", etc. was confusing me.

I've noticed, that:


The Detective Boys call Conan "Conan-kun", which seems to be perfectly normal, according to Wikipedia
Genta and Mitsuhiko call Ayumi "Ayumi-chan" (according to Wikipedia, this is okay as well, since they are young children)
Hiroshi Agasa calls Ai "Ai-kun"


But:


The Detective Boys (including Conan) call Ai always "Haibara-san"


Why do they treat Ai, like she was an adult? Genta, Mitsuhiko and Ayumi can't know, that Ai is actually an 18-year-old woman.
To expand my comment, it's because she's a transfer student and always act mature



Ayumi tried to call her Ai-cahn several times but too afraid to do so



But in the end, she was able to call he Ai-chan, but when the other called her that, she refused



Pictures taken from Case Closed Chapter 398 and 400