1Very young, little — describing someone (or something) young in age or having a childlike, innocent appearance.
彼は年の割に幼く見えると言われる。
He is said to look young for his age.
赤ちゃんは目が大きくて幼い表情をしている。
The baby has big eyes and an infantile expression.
幼いころからピアノを習っていた。
I studied piano from a very young age.
幼い子供たちが公園で楽しそうに遊んでいる。
Young children are playing happily in the park.
2Childish, immature — describing thinking or behavior that is undeveloped or shows a lack of experience and judgment.
そんなやり方は幼くて、現実的ではない。
That approach is childish and unrealistic.
彼の意見はまだ幼いが、将来変わるかもしれない。
His opinion is still immature, but it may change in the future.
大人なのに幼い行動をするのは恥ずかしい。
It's embarrassing for an adult to behave childishly.