So, insulting isn't the best term. Uncivil is. But you asked for an explanation.
Civil discourse is something that, quite frankly, is very hard to explain clearly and concisely. Even people who study parliamentary procedure, debate, public service, international trade, and other topics that rely on cooperation and civil discourse in their daily life simply learn what civility is over time.
While "kid" may not be insulting on its own, I pointed out two reasons it's insulting in the context of your post - namely that it assumes their age based on their behavior (an insulting assumption), and that it stereotypes children to behave in a negative way (a negative stereotype).
You cannot know from previous comments that he is a child unless they themselves have admitted such - which I am almost positive they did not. You do not get to utilize your assumptions to justify themselves.
I'd also like to point out what concerns me about your post here - namely that you have a tendency to use "buzzwords" instead of thoroughly explaining yourself. The use of words such as "culture of offense and victimhood" is such that you are, once again, using stereotypes and attempting to discredit and/or argue against something simply because of those stereotypes. That is a prime example of what is not civil - attempting to justify your opinions/statements based on things such as stereotypes and assumptions, and telling others that their "culture" is wrong. This is especially true when you use terms such as "offense" - which is a perfectly legitimate belief/feeling/emotion - in a negative connotation. Saying that there is a "culture of offense" is a way you are attempting to claim you are correct because everyone is "offended" by things they should not be offended by. This is not a civil way to talk.
You go on to claim people are "thin skinned". You go on to imply that all "youngsters" are thin skinned. We are back once again to stereotypes. Would you call a coworker in your research lab job a "kid" simply because they say your machine was calibrated incorrectly? Just because you disagree with them, would you say "well, you just have a culture of perfection"? You wouldn't, right? The reason you would not go to such depths in a discussion at work is because your workplace would likely see that as reason to reprimand you for improper conduct to a coworker - i.e. it's not an example of civil discourse. There is absolutely no reason that this is limited to youngsters. While you may very well be correct that adults will tend to talk more obscenely/frankly with each other in a casual context, in the context of a "formal" discussion with someone you do not know, adults will be civil with each other.
There was furthermore not any abuse of the system here. While he certainly may have been reporting comments for any reason, the moderators would not have removed your comment had it not been incivil. Your comment was removed because it violated the rules - not because any person reported it - and in fact you have no way of knowing if a moderator removed your comment before or after a report. Moderators are not limited in their power on the boards to posts that are reported. They can and will remove any post they see that violates the rules.
To conclude this, I understand completely that you view the boards as a "casual" communication platform such as texting a friend or chatting with friends while playing the game. That is not what discussion boards/forums are. They are simply that - forums. The etymology of the word "forum" to be used to describe a website for discussion comes from the use of the word to describe meeting places for discussion, policy making, and presentation of ideas in ancient cultures - especially Rome. The prime rule in these places was that everyone remain civil - which is why they were effective as meeting places - and that if you didn't have anything new to contribute, you did not "spam" the forum with useless comments. If you're looking for something other than a forum - a place devoted to civil and serious discussion, then I encourage you to seek out and find those places. There is absolutely nothing wrong, culturally or otherwise, with the fact that other people choose to utilize this site as a forum - and it is not because of any "culture of offense and victimhood" that rules are enforced here.
