This is the 8251st most frequent Japanese word.
幼稚
"幼稚" means "childish" or "immature."
Here, "幼稚" is used as an adjective to describe the immature actions of the child, indicating a lack of development or sophistication in behavior.
あの子の行動は幼稚である。
That child's behavior is childish.
In this sentence, "幼稚" describes how his words or tone seem immature or lacking depth, used with the copula "に" indicating a manner or appearance.
彼の言うことは時々幼稚に聞こえる。
What he says sometimes sounds childish.
"幼稚" here is used as an adjective to describe ideas or thoughts, characterizing them as simple or immature, which is correlated with youth in the context.