How old was David when he fought Goliath?
The Bible never gives David's exact age. It calls him a youth, so most estimates place him in his mid-to-late teens — but that is an inference, not a stated fact.
Scripture does not record a number. In 1 Samuel 17 David is repeatedly described with words for a young man: Saul tells him, "thou art but a youth" (1 Samuel 17:33), and Goliath "disdained him: for he was but a youth" (1 Samuel 17:42).
Because David was old enough to tend sheep and carry food to his soldier brothers, yet still too young for battle in Saul's eyes, many scholars estimate he was somewhere between about 15 and 19. Anyone who gives you a precise age is guessing — the text simply doesn't say.
Scripture references
1 Samuel 17:33
1 Samuel 17:42
Scripture is not explicit; the common teenage estimate is an inference.