It's because everyone here is unoriginal and can't think of a better excuse for being upset at someone other then to label that person by their age. It's synonymous to racism and other generalization except 95% of the populace here would fit in that category anyways so odds are if you shout out "Kids are stupid" you'll be simultaneously saying you're stupid as well.
It's also one of the internet's most popular weak burns that everyone uses like a comma along with "kill yourself the way I just described" and other weak FINISH HIM insults.
You're a child (most popular apperently), go ride on a bike and accidently hit a small rock cauing some strange momentum of physics that send you hurling into a transport truck going twice the speed limit (kill yourself the way I just described). loving NOOB! (Finish him)
But yes. You are right. However, when I tend to say "You're childish." I don't mean they are of a certain age. I mean they act like what is suspected of the average 8 year old of todays society, regardless of how old they really are. I personally don't like to label people by anything at all, and I think I may have been recently due to some complications of some issues (Issues not relevant to the topic).
I've met a good share of intelligent children, stupid children, and same for adults and such. Which is why I think I should stop saying such things. No one should be categorizing a people. We are all in each our own selves. (Except the few in our world who choose to be someone else. Either way that's their choice.)