Jessee had 8 sons (1 Sam 18:12). The oldest 3 were 20 years old and older according to Numbers 1:3. At the youngest, the 3 oldest sons would have been 22, 21, and 20 (unless they were triplets and all 20 years old). The 4th oldest son didn’t serve in the army, which means at the oldest, he would have to be 19. That leaves the 5 sons who didn’t join the military to be 19, 18, 17, 16, and 15 assuming that David’s mother had 1 son a year. It’s also a possibility that David had elder twin brothers, which would potentially make him older than 15.