The story of "This Is What It Means to Say Phoenix, Arizona" says that when Victor and Thomas Builds-the-Fire were seven-years-old, Thomas told the story about Victor's father watching TV all night until it vanished into white noise, like the way he'd like to vanish and never be found because he had a "weak heart" and was afraid of his family and of Victor. Then the story says that when Victor and Thomas were ten they shared a bike and rode it to the Fourth of July Fireworks display where Thomas remarked that he doesn't know why they celebrate the Fourth "it ain't like it was our independence everybody was fighting for." At the end of the evening, after Thomas tells a story of how the two became very brave warriors, the boys go their own ways home, "both laughing all the way."
The next flashback in the story tells that when they were fifteen-years old and, for a long time, no longer friends, Victor got very drunk and for no reason seriously beat Thomas, who was rescued by powerful Norma Many Horses. This is the night that Thomas's stories stopped so that at the time of the story, the event of Victor's father's death, Thomas only told the same old stories over and over. When Norma rescued him from the beating being given him for no apparent reason by his former best friend, Thomas closed his eyes but "no stories came to him, no words or music."
This is all the evidence that is provided in the story about Victor's father's departure. He was there when they were seven. Thomas told a story saying Victor's father wanted to (not was presently going to) leave on a motorcycle and never be found. When they were ten, they laughed and shared a bike and a story about becoming warriors and being called very brave. They both went home laughing, which indicates that Victor's family was intact and he was content and happy. When they were fifteen, Victor, no longer friends with Thomas, turned on him and beat him up, and Thomas lost his stories in grief.
The most logical analytical conclusion is that Victor's father left during the time between Victor's tenth and fifteenth years. The probability falls toward his departure being closer to age fifteen than ten because if they had been ten or eleven, Victor would most likely still have been innocent enough to still believe in Thomas's stories and to turn to Thomas for comfort. If, on the other hand, Victor were thirteen of fourteen, he would have been more likely to react in anger at the memory of Thomas's story and hold Thomas responsible for what he might have thought of as a prophesy, thus blaming Thomas. Victor's father's departure would therefore force him to grow up too soon in one way by robbing him of his belief and his peace of mind and driving him into a feeling of enmity with the world as represented by Thomas Builds-the-Fire.