as a non american, can i ask why in the karnchonlatee fic chon is referred as “kid”?
Primarily because the fic is written from Karn's point of view and I use it to show his characterization.
Karn refers to Chon as 'kid' for the same reason he refers to Chonlatee's friends as "college dipshits" - he is very arrogant and quite a bit older. So it's in his asshole personality to use diminutives or insults for people younger and "lesser" than him, in order to reinforce his superiority to them.
Karn mostly uses kid/boy for Chonlatee in the first fic, when he doesn't know Chonlatee's name/doesn't care about it. As he develops both feelings and respect for Chonlatee, he tends to use a more specific and fond appellation (i.e. 'the little minx'). These pet names will still often include a diminutive (little) to reinforce Karn's arrogant nature, though they will change when he is particularly impressed by Chon (sly, clever etc.). He still uses 'kid' when Chonlatee is being particularly impertinent.
In a more general sense, "college kid" is quite a common phrase in America, used to refer to anyone in college (~18-22), so 'kid' is not exclusively used for minors - just like boy or boyfriend can be used for people in their 20s and even older. 'Kid' gives the sense that you are looking down on the other person, and consider them beneath you in terms of maturity/sophistication. "Those noisy-ass kids" is a complaint that can easily be said by someone in their thirties about a group of early twenties, especially when followed by "I was never such an idiot when I was that age".
I hope that answers your question!