unadult

adjective

un·​adult ˌən-ə-ˈdəlt 

-ˈa-ˌdəlt

: not adult : not characteristic of or befitting an adult

unadult behavior

There is something profoundly unadult in all Bogdanovich's descriptions of women.

—Lynn Barber

Word History

First Known Use

1944, in the meaning defined above