David was likely a teenager, approximately between 13 and 16 years old, when he killed Goliath. The Bible does not state his exact age, but several clues help estimate it:

• Saul refers to David as a "youth" or "boy" when David volunteers to fight Goliath, indicating he was not yet a man of fighting age

• According to Israelite law, men had to be at least 20 years old to serve in the army, and David was not yet enlisted, while his three oldest brothers were already soldiers

• Jesse, David's father, had eight sons, with David being the youngest. Assuming roughly one year between births, David would have been around 13 to 16 years old at the time of the battle

• Some sources suggest David could have been as young as 13 or as old as 19, but the consensus leans toward early to mid-teens

In summary, David was most likely between 13 and 16 years old when he defeated Goliath, making him a young shepherd boy rather than a seasoned warrior