1A noun usage referring to early childhood or infancy itself; rarely used alone in modern Japanese, where the na-adjective sense below is far more common.
2Childish or immature in thinking, attitude, or behavior; commonly used to critically describe a person, opinion, or work.
あの漫画のストーリーは幼稚すぎると批判された。
That manga's story was criticized as being too childish.
幼稚な言い訳はもうやめてほしい。
I wish he'd stop making such childish excuses.
彼の対応はまるで子供のように幼稚だった。
His response was as childish as a kid's.