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"
- 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.