The word "kid" and what it means to its users

[Wynnyelle]

I use it in the way that you are supposed to use it, to address a child.
Sometimes though, I will use it to address somebody who wouldn't normally be considered a child if they are acting like one. Not only does it make them angry, it makes them see how dumb they're acting.

[Puck]

It's placed upon anyone with a differing opinion than your own.

[Athorha]

you done son!

[Arrowspace]

Most of the time, the word is used in argument as an insult. Usually it doesn't work very well.

I'll mostly use it in a descriptive sense. If some random guy in a pug is whining about loot/dying/whatever and then ragequits, I could follow up with the observation "gee, that guy sure was acting like a kid." Equivalent to calling someone a "baby" if they're crying about something trivial, I reserve "kid" for people that rage about trivial things.

Sometimes, if I want to have a little fun, I'll use it to get someone to rage even further. Last week in LFR, I was tanking Morchok on my death knight, and after the fight, I calmly noted that there were some DPS under both of us tanks on the DPS meter (and I'm only 378 geared). One of those DPS, a Frost Death Knight, started raging at me in tells over it. He spent the rest of the instance mocking me in tells if he beat my BLOOD DK on the meter (!?) and ninja pulling to make it harder for us to kick him. He was generally acting like, well, a kid. I didn't really reply to him except to voice how much his fury was entertaining me (mostly because the game was telling me "player not found" when replying), but that could have been one of those situations where I could have called him a kid just to watch him explode with anger. And it would have been great fun.

I won't use it as an honest insult, because you never know the age of the person on the other end. But it's a fun word to use when you know they're going to rage hard at you for using it.

[Senka]

Part of it is because real life age does not matter one tiny bit on the Internet. I think it can actually be a good and reasonable insult when used properly and appropriately. Unfortunately it's yet another abused word online and just used by anyone that thinks that insult alone automatically wins them any argument without looking like they lost their temper and makes them more mature when it's really the opposite.

In much simpler terms: It's a classic case of something good turned bad by new people joining the Internet misunderstanding it.