Meaning of “adult” - Learner’s Dictionary

adult

noun [ C ] us uk /ˈædʌlt, əˈdʌlt/
Extra Examples
Children under 10 years of age must be accompanied by an adult.Children learn by observing adults.There'll be three adults and sixteen children.She prefers the company of adults.Why can't we discuss this reasonably, like adults?

A1 a person or animal that has finished growing and is not now a child

(Definition of “adult noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)