Have you ever heard someone be accused of being "childish", or been accused of that yourself? You probably have, because that's an extremely common insult. It's used when you have political opinions that other people disagree with (anarchy, for instance, is incredibly childish!), it's used when you dare to defy authority and stand up for yourself (don't be childish! Just do it!), it's used when someone expresses their emotions openly (it's childish of you to be angry/hurt/upset!).
This is so insulting to so many people, on so many levels!
When people use that word, when they say "childish", what they mean is that anything a child comes up with, any thought, opinion, emotion, is absolutely worthless and discard-able. To be a child is to have nothing of worth to show for yourself. It's an expression of ageism at it's very worst!
So when someone tells me that I'm being childish, they're not only insulting children everywhere, they're also telling me that my opinions are worthless. That they're short-sighted, uninformed, unimportant, and simply not worth paying any attention to.
I think that's one of the things that makes me the absolute angriest when someone says it to me. And I've heard it WAY more often than I'd like!
I've heard that the most marginalized members of society are the young and the old, and I don't have much trouble believing that. Ageism and disrespect are so very common, that I find it seriously discouraging sometimes.