Very young; little — describing someone or something that is young in age or 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.
Childish; immature — describing thoughts or behavior that are undeveloped or juvenile.
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.