Infancy (noun): historically used to refer to early childhood or infancy; this noun usage is limited and less common in everyday speech.
この概念はまだ幼稚の段階にあると考えられている（文語的表現）。
This concept is still considered to be at an infancy stage (formal/literary expression).
Childish; infantile; immature — used (as a na-adjective) to describe thoughts, behavior, or expressions that are immature or unsophisticated.
そんな冗談は幼稚だ。大人らしい対応をしなさい。
That kind of joke is childish. Behave like an adult.
彼の説明は論理的ではなく、どこか幼稚に感じられた。
His explanation wasn't logical and felt somewhat immature.
幼稚な考えを改める必要がある。
There's a need to change such childish thinking.