We know he was also underage, but the burning question is whether he crossed state lines with the device?
Most likely David did not cross state lines, as both Bethlehem and the valley of Elah are in Judah. But the actual ownership of the land was in dispute, as the Jews ownership rests completely of the claim that the land was given to their forefather Jacob by Issac by Abraham by God, and was therefore solely thiers through inheritance. Other children of Abraham and Issac have picked up the name and the cause of the displaced Philistines, and are currently disputing that claim in the United Nations.