Final answer:
The specific time when Mason and Benny Kilmartin became best friends is not detailed in the book; the story addresses Mason's life following Benny's death.
Step-by-step explanation:
In the book The Truth as Told by Mason Buttle, the timeframe of when Mason and Benny Kilmartin became best friends is not explicitly stated. The story focuses rather on Mason's life after Benny's tragic death and how Mason copes with the loss of his friend as well as the suspicions and accusations that come with it. To fully understand Mason and Benny's friendship, one would explore the narrative and how Mason reflects on their past experiences together.