How old was David when he fought Goliath?

No number in the text

1 Samuel measures Goliath to the span and weighs his armor in shekels, but it never says how old David was in the Valley of Elah. The first age the Bible gives him is thirty, when he becomes king (2 Samuel 5:4). Everything earlier has to be worked out from clues.

The clues in 1 Samuel

• The youngest of eight. Jesse “had eight sons” and was already “an elderly old man in the days of Saul” (17:12). David is the last (17:14).
• Not yet a soldier. Only the three oldest, Eliab, Abinadab and Shammah, had gone to war with Saul (17:13). David shuttles between the camp and his father’s sheep, and arrives with grain, bread and ten cheeses (17:15–18). Israel counted its fighting men “from twenty years old and upward” (Numbers 1:3), which many readers take as a hint that David was under twenty.
• “But a youth.” Saul says it to his face: “you are but a youth, and he a man of war from his youth” (17:33). Goliath sneers for the same reason (17:42).
• Lion and bear. Yet David tells Saul he has caught a lion by its beard and killed it (17:35). No small child does that. Saul even dresses him in his own armor (17:38).

What “youth” means in Hebrew

The word is na’ar, and it stretches. It is used of Joseph at seventeen (Genesis 37:2), of Joshua when he is already Moses’s aide (Exodus 33:11), and of Absalom, a grown man leading a revolt (2 Samuel 18:5). Solomon, newly crowned, uses it of himself: “I am just a little child” (1 Kings 3:7). So na’ar says David was young and untested beside a lifelong warrior, not that he was a little boy. Watching him go out to fight, Saul uses a plainer word, elem, a young man: “Inquire whose son the young man is!” (17:56).

Counting back from thirty

David is thirty when he begins to reign. Before that come his years at Saul’s court, his marriage to Michal (18:27), years on the run with his band of outlaws, and “a full year and four months” among the Philistines (27:7). The text dates none of these stretches, but together they suggest a decade or more. Count back, and the giant-killer lands in his teens. That is why most commentators picture an adolescent, somewhere between about fifteen and twenty: young enough to be dismissed, old enough to be dangerous.

Two portraits of David

There is a puzzle behind the youth. In chapter 16, before the duel, one of Saul’s servants describes David as “a mighty man of valor, a man of war” (16:18), and David becomes Saul’s armor bearer (16:21). Yet as David goes out to fight, Saul asks, “Abner, whose son is this youth?” (17:55), as if he had never met him. The oldest Greek translation, the Septuagint of Codex Vaticanus, is much shorter here: it lacks David’s arrival with food for his brothers (17:12–31) and Saul’s questions about who he is (17:55–58). Many scholars think two tellings were woven together, one of a young warrior at court and one of an unknown shepherd boy.

The boy won the popular imagination. Donatello’s bronze David, in Florence, is a slim boy in a brimmed hat; Michelangelo’s marble is a young man at full strength. For the giant’s side of the duel, see how tall Goliath was.