How old was David when he fought Goliath?

[tonychanyt]

You needed to be 20 years or older to join the army (Numbers 1:3).

Saul's servants knew about David in 1 Samuel 16:

18 One of the servants answered, “I have seen a son of Jesse of Bethlehem who knows how to play the lyre. He is a brave man and a warrior. He speaks well and is a fine-looking man. And the LORD is with him.”

Saul hired David as a musician and not as a soldier probably because he was under 20 of age.

David was the youngest of 8 sons of Jesse (1 Samuel 17:12–14). David's three oldest brothers were Saul's soldiers in 1 Samuel 17:

13a The three oldest sons of Jesse had followed Saul to the battle.

Later, David volunteered to fight Goliath, but

33 Saul said to David, “You are not able to go against this Philistine to fight with him, for you are but a youth, and he has been a man of war from his youth.”

He was old enough to marry Saul's daughter (1Sa 18:27).

2 Samuel 5:4 mentioned that David was 30 years old when he became king, which happened after Saul's death, some years after the fight with Goliath.

David was probably in his late teens when he fought Goliath. I'd guess 18.