We begin with Green Lantern Hal Jordan, who first appeared in 1959, which is 58 years ago.
Well, the first question we have to answer is, how old was he in 1959?
(Bear in mind, that all of this is based on my admittedly incomplete knowledge of the character. I am open to correction.)
I don't know that an exact age was ever given, if it was, I never heard about it. I always took him for about 30. The way he was drawn, he didn't look to me like he could be much younger than that. Plus, while I don't know much about becoming a test pilot, minutes of internet research has led me to believe that one doesn't simply roll out of flight school and become the sort of test pilot that Hal Jordan was shown to be. I had the impression that he'd been in the USAF, so presumably he did at least a minimal tour of duty, and spent sometime establishing himself as a civilian test pilot, so I figure that 30 is a reasonable estimate.
In that case, Hal Jordan would be 88 now.
So, what would he be doing? I like to think he would have had a long, noteworthy career in the Green Lantern Corps and the Justice League of America. Undoubtedly, he would eventually have been offered a spot in the Guardians; Honor Guard, but it wouldn't surprise me if he turned them down for more active duty.
I expect he would be retired by now, although I can imagine an older Hal taking a hand in training younger or less experienced Lanterns. I expect these would be like the children he never had - I just can't see him as the "settling down" type, somehow. Now, he's living somewhere in comfortable solitude, occasionally getting together with Barry and Ollie and maybe some of the others for a beer, and listening as they tell him what their grandkids did. It's become a custom that when whoever is the current Green Lantern for Sector 2814 is out Earth way, they make it a point to stop by if they can, and fill him in on the latest Corps gossip and scuttlebutt.