Very Young
Childish
い adjective
The kanji means infancy. The adjective version describes someone who is very young, like little kids in their early childhood or babyhood!
Depending on the context, it can also describe behaviors or ideas that are childish or immature, often with a negative connotation.
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
When you were very young and childish you were sent off to do prayers to ask for forgiveness. One of the things you had to do was sing "hosanna (おさな) in the highest."
あの子、まだ幼いのに、かわいそうだね。
I feel sorry for that little girl, she's still so young.
病院で、迷子になっている幼い男の子がいました。
There was a little boy who got lost in the hospital.
お兄ちゃんの方が、考え方がちょっと幼いんだよね。
The older one has a slightly more childish way of thinking.