Rubin Pritchard is two years older than Billy; his brother Rainie, the youngest Pritchard, is the same age as Billy. That would make Rubin about fifteen or sixteen when he died.
When the story begins, Billy, the narrator, is a child of ten. Billy is almost eleven when he begins saving for the two hounds he has always wanted, and it is two years before he has enough money collected to send for them. Billy gets his hounds when he is about thirteen, and he has them for a little over a year.
Billy's hounds have already established a reputation for themselves as excellent coon dogs when Rubin Pritchard challenges Billy with a bet that his "blue tick hound can out-hunt both of them". Rubin is described as
"...two years older than (Billy), big and husky for his age. He never had much to say. He had mean-looking eyes that were set far back in his rugged face...smoky-hued and unblinking, as if the eyelids were paralyzed...Ranie (is) the youngest, about (Billy's) age".
Since Billy got his hounds at thirteen, and at the end of the story, when his dogs have died, he is only fourteen, the accident when Rubin is killed has to have taken place somewhere in that timeframe. If Rubin was two years older than Billy, he would have been fifteen or sixteen when he died (Chapter12).