This is the 10660th most frequent Polish word.
dorosłym
"Dorosłym" means "adult" in English.
The word 'dorosłym' is used here as an adjective in instrumental case, describing the state of becoming an adult.
Mając osiemnaście lat, stajesz się dorosłym.
At the age of eighteen, you become an adult.
In this sentence, 'dorosłym' is used in the instrumental case as a predicate nominative, describing the subject of the gerund 'bycie' (being).
Bycie dorosłym wiąże się z odpowiedzialnością.
Being an adult involves responsibility.
Here, 'dorosłym' is used as a noun in instrumental case, referring to the individual participated in the conversation.