I know in biblical Hebrew that age of boys and girls was often determinable by the word the author used. In Judges 8:20 we read about Gideon's son Jether being afraid to murder Zebah and Zalmunna because he was still a "young man".
How old would Jether have been?
Judges 8:20-21 So he said to Jether his firstborn, “Rise and kill them!” But the young man did not draw his sword, for he was afraid, because he was still a young man. Then Zebah and Zalmunna said, “Rise yourself and fall upon us, for as the man is, so is his strength.” And Gideon arose and killed Zebah and Zalmunna, and he took the crescent ornaments that were on the necks of their camels.<