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.
もり田さんには、年下のかの女がいます。
Morita has a younger girlfriend.
コウイチはアーノルドシュワルツネッガーより三十八才年下だ。
Koichi is 38 years younger than Arnold Schwarzenegger.
あの子は年下なのにとても大人びている。
Even though that kid is younger, he’s really mature.