Meaning of “immature” in the English Dictionary

"immature" in British English

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immatureadjective

uk /ˌɪm.əˈtʃʊər/ us /ˌɪm.əˈtʊr/

C2 disapproving not behaving in a way that is as calm and wise as people expect from someone of your age:

Stop being so silly and immature, Chris!
She's rather immature for her age, don't you think?

disapproving not having much experience of something:

politically immature

specialized biology not yet completely grown or developed:

While the animals are still immature, they do not breed.
immaturity
noun [ U ] uk /ˌɪm.əˈtʃʊə.rə.ti/ us /ˌɪm.əˈtʊr.ə.t̬i/

physical/emotional immaturity

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

"immature" in American English

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immatureadjective

us /ˌɪm·əˈtʃʊər, -ˈtʊər/

not completely developed physically, mentally, or emotionally, or lacking the expected type of responsible behavior for your age:

Some of the boys in my class are so immature – they’re always doing silly things.
immaturity
noun [ U ] us /ˌɪm·əˈtʃʊər·ɪ·t̬i, -ˈtʊər-/

physical/emotional immaturity

(Definition of “immature” from the Cambridge Academic Content Dictionary © Cambridge University Press)