Well, we can infer some things from his parents' ranch and his bedroom: that his early years were lonely and isolated and bleak, that he dreamed of being a cowboy (the little statue) and that he probably spent time looking out the window and planning how he would escape. His dad was a good rodeoer, and though he never taught Jack a thing, Jack somehow managed to learn it anyway -- he learned to rebel against authority and figure out a way to improve his situation. We are mostly left to imagine how he came to terms with his sexuality, but his ability to reject his dad's authority probably helped him feel more about comfortable breaking society's "rules." From a completely pragmatic perspective, I'd say the fact that we don't hear much more than that about Jack's adolescence means it wouldn't add to our understanding of his character or the story, so I'm assuming it was fairly uneventful.
Hearing about Ennis' earlier years -- parents' death, unfinished high school, losing the ranch, $24 in a coffee can, siblings getting married and leaving him behind -- does add to our understanding of his character. He's used to being abandoned, he's used to being poor, he's used to accepting whatever life hands him without complaining or hoping for improvement. His teen years were probably even harder than Jack's, which sets up the economic inequality that follows them all the way through the movie.
On the other hand, we don't hear much about Ennis' relationship with his father at first, beyond a couple of offhand, positive comments. This sets us up for shock when the Earl story comes along. We are forced to figure out for ourselves how Ennis felt about his father despite Earl, as well as to imagine what it must have been like on a day-to-day basis, being a gay kid with that kind of dad. But it's not surprising that, growing up with a dad scary enough to be capable of torture/murder, Ennis is less willing to rebel against authority.
