infancy; very young age
Rare as a standalone modern noun for the period of infancy or early childhood; modern Japanese more often uses words such as 幼少期 or 幼いころ.
In older writing, 幼稚 can appear as a noun meaning a very young age.
Common evaluative na-adjective for behavior, ideas, excuses, or arguments that seem childishly undeveloped; often critical.
そんな幼稚な言い訳は通用しない。
Such a childish excuse will not work.
彼の反論は感情的で、少し幼稚に聞こえた。
His objection was emotional and sounded a little immature.
rudimentary; underdeveloped; primitive
Used for a method, theory, technology, or field that is still at an undeveloped or early stage; this is less about literal children and more about lack of sophistication.
当時の技術はまだ幼稚な段階にあった。
The technology at the time was still at a rudimentary stage.
I think his way of thinking is still childish.
An excuse that childish won't work.
This technology is still at a rudimentary stage.
幼稚 (a noun/na-adjective) means 'childish/infantile' and is usually more pointedly critical, while 幼い is an i-adjective that can neutrally describe a young child as well as immature behavior.
幼稚園 is the noun for 'kindergarten', while 幼稚 (written with the same first two kanji) is an adjective meaning 'childish' or 'infantile'.
infant-industry argument; theory of infant-industry protection