I always got the feeling that Ennis had already turned 20, while Jack's birthday was in the fall. My understanding is that they met in early June of 1963. I think Ennis's birthday was in the early spring (March or April), and Jack's was probably September. Annie Proulx uses a lot of colloquial language in her writing, and I noticed that she does the same thing in other stories. So "not yet 20" wouldn't necessarily mean that both Ennis and Jack were still 19. They could have both already turned 20, earlier that year, or late the previous year. Or perhaps Ennis was turning 20 in July, and Jack sometime in the autumn. Or Ennis could have turned 20 in the spring, or early that year in winter, and Jack was the one who turned 20 that summer they worked together, (example, Ennis's birthday is in January or February, Jack's is in July) So I came to the conclusion that they were both born in 1943 and Ennis's 20th birthday is either in the spring, a few months before he met Jack, or in July, in the midst of their summer together. Jack, I concluded was probably born in the fall, or summer, so when they meet, he's either just about to turn 20 or several months shy of it. Given what Lureen told Ennis at the end, that Jack was still 39 when he died, it implies that Jack's birthday wasn't in the summer, since he most likely dies in July or August of 1983 a few months before he would have turned 40, hence I would say that Jack was born in September or October, and Ennis was born earlier the same year in the spring or summer. The age difference between them was most likely only 3-6 months, with Ennis being the "older" one.