This is the 4412th most frequent Russian word.
взрослые
Adults.
Here, 'взрослые' is used as a noun meaning 'adults', referring to mature individuals.
Взрослые часто берут на себя ответственность за семью.
Adults often take responsibility for the family.
'Взрослый' is used as an adjective meaning 'adult', describing a person who has reached maturity.
Теперь ты взрослый человек и можешь принимать решения самостоятельно.
Now you are an adult and can make decisions independently.
Here 'взрослые' functions as an adjective and is used to describe books intended for adults.