He could have been involved in the fighting despite being quite young--a 13-year-old who had some training and was big for his age might easily have found himself being treated as an adult when Guardia started running out of soldiers. And a resistance movement of some sort might have continued to exist for years after the end of the actual conquest. He might also have lost family or friends, and be looking for revenge, even if he wasn't able to participate in the actual fighting...
Hmmm. There's a fanfic in that, I think. Let's see, now...
The Porrean general looked up from his paperwork as he heard the sound of something crashing against the door. As the noise came again, he began to scrabble at the underside of his desk, seeking the latch the would open the secret passageway in the far wall and permit him to escape his office without passing through the noisy corridor.
He never found it.
There was a third crash, and the door fell from its frame. A huge man stepped into the office, treading carefully over the splintered wood. He wore nothing but a loincloth and a helmet, so the general could see that his muscular limbs were slick with the sweat of recent exertion. In his hand, the giant held a naked sword, with blood dripping from the tip of the blade. The helmet turned as though its owner were scanning the room for...something.
"W-who are you?" the general quavered, shrinking back into his desk chair.
The half-naked man froze in the act of raising his sword to strike the general down, and words issued from under the helmet:
"HELLO, MY NAME IS ZOAH MONTOYA. YOU KILL MY FATHER. PREPARE TO DIE."
