Meaning of “immature” - Learner’s Dictionary

immature

adjective uk us /ˌɪməˈtjʊər/
BEHAVIOUR

not behaving in a way that is as wise and calm as people expect from someone your age:

Some of the boys are quite immature for their age.
NOT DEVELOPED

not completely developed:

immature cells
immaturity noun [ U ]

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