Saying the word "Kid" to another player should be on the same lines of saying "Retard".
They're both terms that mean the same thing: To belittle a player and making them feel like they're less important than you because you're underaged, compared to "retarded", meaning that you're still less important and that your thoughts mean nothing because you're mentally handicapped.
Seriously though, I get really fucking triggered when people use this word against me because I know they get away with it. They can call me a racial slur or mentally unstable all they want, it's the words that "aren't" toxic that piss me off.