John Stewart's age can vary depending on the specific storyline and continuity in DC Comics. Generally, though, he's often depicted as being in his 30s or 40s.
I don't think Vixen dated John Stewart in the comics. The comic plots didn't show such a romantic connection between them. Maybe in some alternate universes or fan-made stories, but not in the mainstream comic canon.
Some dedicated Young Justice or Constantine fan forums might also have links or even their own fanfictions. For example, Reddit has sub -reddits related to these topics where fans might share their own works or recommend good fanfictions they've read.
One great story could be 'Constantine's Influence in Young Justice'. It might show how Constantine's dark magic and street - smart ways impact the young heroes. Maybe he teaches them some unorthodox ways to deal with magical threats that the League of Justice doesn't usually use.
One of the great John Stewart Green Lantern stories is 'Mosaic'. It shows his leadership and the complex relationships within the Green Lantern Corps. He had to deal with a shattered world and bring its pieces together, both literally and figuratively.
One key element is his military background as a Marine. It gives him the discipline and combat skills. Another is his strong sense of justice which made the Green Lantern ring choose him.
John Stewart's origin is quite an interesting one. He was living his life as a Marine, having his own set of skills and experiences. Then, out of the blue, he got the opportunity to be a Green Lantern. The Green Lantern ring chose him. His origin story involves not just getting the power but also his growth. He had to face challenges from both within the Corps and from external enemies. His origin story is intertwined with his development as a character, his relationships with other Green Lanterns, and his struggle to protect the universe, which makes it a great and complex origin story in the DC Comics world.
Well, it could be really interesting. They might be amused by some of the creative interpretations in the fanfiction. Some might find it strange to see their own characters in different scenarios that the fans have come up with.