He who is both in his infancy and immature is probably childish and immature.

When used as an adjective, 幼稚 describes negative immaturity in someone's behavior, remarks, or ideas. However, in compounds like 幼稚園 (kindergarten), it simply refers to the early childhood stage without any negative tone.

This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.