Younger
noun, の adjective
When you have someone who has years below your own, this person is younger than you.
としした
The reading for this one is kind of strange. It is the kun'yomi reading for both of the kanji. You know the kun'yomi readings from earlier, back when you learned the vocab 年 and 下.
このクラスは年下の子が多いです。
This class has many younger children.
年下の男の子とデートに行きました。
I went on a date with a younger boy.
私は、姉よりも十才年下です。
I am ten years younger than my sister.