"Are you an adult?"

Translation: Czy ty jesteś dorosły?

[Bob20020]

Is this in the instrumental case? Isn't that needed here?

[Djenthallman]

I think that's an adjective, not a noun.

Instr. is dorosłym.

[Emwue]

You are right, „dorosły” here has adjectival role and therefore requires nominative.

Part 3

[neilymon]

I chose dorosli and it accepted my answer saying I made a typo - I probably should’ve been marked wrong ...

[KlaraDahlb]

Why is not dorosłym the correct answer? Why does dorosły work? Is it because there is no noun coming after it?

[Jellei]

"dorosły" is mostly treated like an adjective in Polish. "dorosłym" would treat it as a noun, and while it may not be wrong, I'd say it sounds rather strang here.