I wrote "I am mature and he is not!" and was counted incorrect, but as I'm not sure if that is right, I didn't report it. "An adult" suggests that מבוגר is being used as a noun, while my translation uses it as an adjective; maybe there's a need for a copula in my case? Maybe I'm just confused.
Brandon, whether or not מבוגר is used as an adjective or a noun, the meaning of the word is usually either fully grown or at an age in which society acknowledges you’re no longer a child.
In a scientific world, mature similarly means the state of being fully developed physically, but psychologically and colloquially, people use mature to mean knowing how to and/or behaving appropriately. Because the word mature has this possible meaning, your sentence can mean: I am an adult who is mature and he is an adult who is not mature!
At its most basic meaning, that is not what the Hebrew says, so that’s my guess as to why Duolingo did not accept your answer.