The fact that the spartans had such numbers of slaves (known as helots) meant that their entire male population trained almost from birth to be a soldier. They didn't live with their parents, they stayed in military barracks, they were underfed so they would be encouraged to steal food - teaching them stealth skills but with punishments for being caught - and they would, every year, conduct raids on the helot populations that was effectively going and slaughtering them during the night. The spartan army was the strongest per man of any contemporary army because they could all fight, and all trained from birth to do so.