I think more people should know ZIM is not a child. The show makes it pretty clear that ZIM is an adult. You can see so from the way he often refers to Dib and his “peers” as children. Zim himself is calling them children, implying he is significantly older and not a child anymore.
Not to mention it has been mentioned that ZIM is at least 50+ years old (while we don’t know how old irkens live, that still gives him 50+ years of life experience) (the rest of his classmates are likely 11-12 years old)
I’ve heard people say that Zim is “child-coded”, but that does not make him a child. Zim can be childish, while also being an adult.
You can have an adult character who is childish, but that does not make them a child
(i.e. Spongebob)
There are also several posts where the creator mentions that ZIM is an adult. I know a lot of people don’t take everything Jhonen Vasquez says head on, but I think when we combine his words with the implications presented in the show then it’s pretty safe to say that ZIM was always written to be an adult.