1 Samuel 17:14

David was the youngest. The three oldest joined Saul's army.

1 Samuel 16:11

Are these all the sons you have? Samuel asked. There is still the youngest one, Jesse answered. He is tending sheep. Samuel responded: Send someone to get him. We will not continue until he gets here.

Genesis 25:23

Jehovah said: Two nations are within you. You will give birth to two rival peoples. One will be stronger than the other. The older will serve the younger.

Treasury of Scripture Knowledge

Summary

And David was the youngest: and the three eldest followed Saul.

Bible References

The youngest

1 Samuel 16:11
Are these all the sons you have? Samuel asked. There is still the youngest one, Jesse answered. He is tending sheep. Samuel responded: Send someone to get him. We will not continue until he gets here.
Genesis 25:23
Jehovah said: Two nations are within you. You will give birth to two rival peoples. One will be stronger than the other. The older will serve the younger.