“Then all Israel gathered together to David at Hebron, and said. ‘See, we are your bone and flesh. For some time now, even while Saul was king, it was you who commanded the army of Israel. Yahweh your God said to you. ‘You shall be the shepherd of my people Israel. You shall be the ruler over my people Israel.’ All the elders of Israel came to the king at Hebron. David made a covenant with them at Hebron before Yahweh. They anointed David king over Israel, according to the word of Yahweh by Samuel.”
Actually, it took many years before David was able to rally the Israelites of the north to follow him, as this biblical author omits all the problems that David had in 2 Samuel, chapters 1-4. Thus this biblical chronicler starts with 2 Samuel, chapter 5, almost word for word in a more idealized portrait story of David. This is the first mention of the prophet Samuel by this biblical author. The elders of all the tribes of Israel came to Hebron. They were the same flesh and bone which is what the men of Judah said, not all the Israelites. Yahweh wanted David to be king over all Israel. Then they made a covenant or treaty with David before Yahweh. Finally, they anointed David King of Israel. There is no mention of how old David was at this time.