That Goliath story is really a kind of mashal, a represents how David was victorious against the Philistines.
Philistines were the first in the region to use and fight with iron implements which gave them arms advantage over Israel. This was the reason they were a trouble for so long. At some point David was able to get hold of the vital information regarding the enemy's advantage and begin to turn the tables.
Meaning of mashal.. that Israel struck Philistine with inferior weapons and by daring, cunning and talent hit them where it really hurt.. then were able to take and use their own technology against them. How did David cut off head of Goliath? With his OWN sword. At this point David had tech in hands. Doesn't need to be small boy.. all that is allegorical.. probably happened much later.
How did he get the Philistine technology? Spies? Good advisors.. OR enemy weapons retrieved after a battle and examined, then send spies to find out about smelting.
Note hierarchy of weapons... unalloyed copper weaker than wrought iron weaker than bronze, weaker than steel. As soon as enemy has steel i.e. strengthened iron you NEED iron tech. If enemy uses inferior preparations of iron you can still beat them if you have access to tin to strengthen the copper (bronze alloy).