Childish
Immature
noun, な adjective, の adjective
He who is both in his infancy and immature is probably childish and immature.
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.
どうして幼稚な喋り方をする日本人女性が多いのですか？
Why do so many Japanese girls speak like children?