John Gill Commentary 1 Samuel 17:14

"And David was the youngest; and the three eldest followed Saul." — 1 Samuel 17:14 (ASV)

And David was the youngest
For the sake of whom this account is given of Jesse and his family, and who after this makes a considerable figure in the camp and court of Saul:

and the three eldest followed Saul ;
as before related, and which is repeated, that it might be observed that they only of Jesse's sons followed Saul; not David particularly, but who was providentially sent to the army at the time the Philistine was defying it.