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! 克里斯，不要这么愚蠢幼稚！

She's rather immature for her age, don't you think? 她都这么大了，你不觉得她显得相当幼稚吗？

Synonyms: childish juvenile puerile

not having much experience of something

politically immature 政治上不成熟

not yet completely grown or developed

While the animals are still immature, they do not breed. 动物发育未全时不会生育。