"Your parents are very young."

Translation: 你的父母很年轻。

[KateinaTuz]

为什么这里要用'的'?

[keyongming]

Because it is close familial relations, many (most?) native speakers would omit 的. 的 creates a bit of distance between the modifier (你) and the thing modified (父母).

[clintack]

“你父母很年轻"？
"你的父母非常年轻“？