Can we talk about the fact that Zim as a human was a child?
(Also, this isn't Professor Membrane. This is his alien clon, so in this swap he is human.)
Edit because I think I should add this: With this post I wanted to point out that Jhonen do and say what He wants, even if this later doesn't have sens, like Zim's age. So We shouldn't too. (Also, Jhonen said that He could make Zim x Gaz canon. He really doesn't care what He do.)
And another thing- Irkens years maybe work for them differently. Maybe that's why Zim as a human has a child's body (This everything depends on how long they live, We only know that Irkens are long-lived.)
+These are just my thoughts and opinions