Military service in Noxus is compulsory, for a minimum of six years; although, it is not uncommon to see someone remain active for up to ten years. Seeing someone remain active in the military for 30 years, however, is an entirely different story; one that winds up with Stark.
From an early age, he had his life cut out for him. All his father cared about was getting him trained for battle and shipped off in the military, never to be seen again. When Stark did end up finally going off to the military, it was nothing like what his father had prepared him for. The brutality of war scarred the young man, so harshly that even now he wakes up in cold sweats remembering it. The only way he can suppress the memories of his first time at war is by creating new ones, so he stays on the service doing anything Noxus High Command requires of him.