Immature
Noun, な adjective, and の adjective
ようち
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?
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.